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Try again with an up-to-date version of D69471 (99317124 was a stale revision). --- Revise the coverage mapping format to reduce binary size by: 1. Naming function records and marking them `linkonce_odr`, and 2. Compressing filenames. This shrinks the size of llc's coverage segment by 82% (334MB -> 62MB) and speeds up end-to-end single-threaded report generation by 10%. For reference the compressed name data in llc is 81MB (__llvm_prf_names). Rationale for changes to the format: - With the current format, most coverage function records are discarded. E.g., more than 97% of the records in llc are *duplicate* placeholders for functions visible-but-not-used in TUs. Placeholders *are* used to show under-covered functions, but duplicate placeholders waste space. - We reached general consensus about giving (1) a try at the 2017 code coverage BoF [1]. The thinking was that using `linkonce_odr` to merge duplicates is simpler than alternatives like teaching build systems about a coverage-aware database/module/etc on the side. - Revising the format is expensive due to the backwards compatibility requirement, so we might as well compress filenames while we're at it. This shrinks the encoded filenames in llc by 86% (12MB -> 1.6MB). See CoverageMappingFormat.rst for the details on what exactly has changed. Fixes PR34533 [2], hopefully. [1] http://lists.llvm.org/pipermail/llvm-dev/2017-October/118428.html [2] https://bugs.llvm.org/show_bug.cgi?id=34533 Differential Revision: https://reviews.llvm.org/D69471
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.. role:: raw-html(raw)
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:format: html
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=================================
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LLVM Code Coverage Mapping Format
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=================================
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.. contents::
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:local:
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Introduction
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============
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LLVM's code coverage mapping format is used to provide code coverage
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analysis using LLVM's and Clang's instrumentation based profiling
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(Clang's ``-fprofile-instr-generate`` option).
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This document is aimed at those who would like to know how LLVM's code coverage
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mapping works under the hood. A prior knowledge of how Clang's profile guided
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optimization works is useful, but not required. For those interested in using
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LLVM to provide code coverage analysis for their own programs, see the `Clang
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documentation <https://clang.llvm.org/docs/SourceBasedCodeCoverage.html>`.
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We start by briefly describing LLVM's code coverage mapping format and the
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way that Clang and LLVM's code coverage tool work with this format. After
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the basics are down, more advanced features of the coverage mapping format
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are discussed - such as the data structures, LLVM IR representation and
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the binary encoding.
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High Level Overview
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===================
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LLVM's code coverage mapping format is designed to be a self contained
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data format that can be embedded into the LLVM IR and into object files.
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It's described in this document as a **mapping** format because its goal is
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to store the data that is required for a code coverage tool to map between
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the specific source ranges in a file and the execution counts obtained
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after running the instrumented version of the program.
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The mapping data is used in two places in the code coverage process:
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1. When clang compiles a source file with ``-fcoverage-mapping``, it
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generates the mapping information that describes the mapping between the
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source ranges and the profiling instrumentation counters.
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This information gets embedded into the LLVM IR and conveniently
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ends up in the final executable file when the program is linked.
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2. It is also used by *llvm-cov* - the mapping information is extracted from an
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object file and is used to associate the execution counts (the values of the
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profile instrumentation counters), and the source ranges in a file.
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After that, the tool is able to generate various code coverage reports
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for the program.
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The coverage mapping format aims to be a "universal format" that would be
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suitable for usage by any frontend, and not just by Clang. It also aims to
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provide the frontend the possibility of generating the minimal coverage mapping
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data in order to reduce the size of the IR and object files - for example,
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instead of emitting mapping information for each statement in a function, the
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frontend is allowed to group the statements with the same execution count into
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regions of code, and emit the mapping information only for those regions.
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Advanced Concepts
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=================
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The remainder of this guide is meant to give you insight into the way the
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coverage mapping format works.
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The coverage mapping format operates on a per-function level as the
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profile instrumentation counters are associated with a specific function.
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For each function that requires code coverage, the frontend has to create
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coverage mapping data that can map between the source code ranges and
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the profile instrumentation counters for that function.
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Mapping Region
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--------------
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The function's coverage mapping data contains an array of mapping regions.
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A mapping region stores the `source code range`_ that is covered by this region,
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the `file id <coverage file id_>`_, the `coverage mapping counter`_ and
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the region's kind.
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There are several kinds of mapping regions:
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* Code regions associate portions of source code and `coverage mapping
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counters`_. They make up the majority of the mapping regions. They are used
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by the code coverage tool to compute the execution counts for lines,
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highlight the regions of code that were never executed, and to obtain
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the various code coverage statistics for a function.
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For example:
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:raw-html:`<pre class='highlight' style='line-height:initial;'><span>int main(int argc, const char *argv[]) </span><span style='background-color:#4A789C'>{ </span> <span class='c1'>// Code Region from 1:40 to 9:2</span>
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<span style='background-color:#4A789C'> </span>
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<span style='background-color:#4A789C'> if (argc > 1) </span><span style='background-color:#85C1F5'>{ </span> <span class='c1'>// Code Region from 3:17 to 5:4</span>
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<span style='background-color:#85C1F5'> printf("%s\n", argv[1]); </span>
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<span style='background-color:#85C1F5'> }</span><span style='background-color:#4A789C'> else </span><span style='background-color:#F6D55D'>{ </span> <span class='c1'>// Code Region from 5:10 to 7:4</span>
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<span style='background-color:#F6D55D'> printf("\n"); </span>
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<span style='background-color:#F6D55D'> }</span><span style='background-color:#4A789C'> </span>
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<span style='background-color:#4A789C'> return 0; </span>
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<span style='background-color:#4A789C'>}</span>
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</pre>`
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* Skipped regions are used to represent source ranges that were skipped
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by Clang's preprocessor. They don't associate with
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`coverage mapping counters`_, as the frontend knows that they are never
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executed. They are used by the code coverage tool to mark the skipped lines
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inside a function as non-code lines that don't have execution counts.
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For example:
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:raw-html:`<pre class='highlight' style='line-height:initial;'><span>int main() </span><span style='background-color:#4A789C'>{ </span> <span class='c1'>// Code Region from 1:12 to 6:2</span>
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<span style='background-color:#85C1F5'>#ifdef DEBUG </span> <span class='c1'>// Skipped Region from 2:1 to 4:2</span>
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<span style='background-color:#85C1F5'> printf("Hello world"); </span>
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<span style='background-color:#85C1F5'>#</span><span style='background-color:#4A789C'>endif </span>
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<span style='background-color:#4A789C'> return 0; </span>
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<span style='background-color:#4A789C'>}</span>
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</pre>`
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* Expansion regions are used to represent Clang's macro expansions. They
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have an additional property - *expanded file id*. This property can be
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used by the code coverage tool to find the mapping regions that are created
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as a result of this macro expansion, by checking if their file id matches the
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expanded file id. They don't associate with `coverage mapping counters`_,
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as the code coverage tool can determine the execution count for this region
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by looking up the execution count of the first region with a corresponding
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file id.
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For example:
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:raw-html:`<pre class='highlight' style='line-height:initial;'><span>int func(int x) </span><span style='background-color:#4A789C'>{ </span>
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<span style='background-color:#4A789C'> #define MAX(x,y) </span><span style='background-color:#85C1F5'>((x) > (y)? </span><span style='background-color:#F6D55D'>(x)</span><span style='background-color:#85C1F5'> : </span><span style='background-color:#F4BA70'>(y)</span><span style='background-color:#85C1F5'>)</span><span style='background-color:#4A789C'> </span>
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<span style='background-color:#4A789C'> return </span><span style='background-color:#7FCA9F'>MAX</span><span style='background-color:#4A789C'>(x, 42); </span> <span class='c1'>// Expansion Region from 3:10 to 3:13</span>
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<span style='background-color:#4A789C'>}</span>
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</pre>`
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.. _source code range:
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Source Range:
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^^^^^^^^^^^^^
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The source range record contains the starting and ending location of a certain
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mapping region. Both locations include the line and the column numbers.
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.. _coverage file id:
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File ID:
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^^^^^^^^
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The file id an integer value that tells us
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in which source file or macro expansion is this region located.
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It enables Clang to produce mapping information for the code
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defined inside macros, like this example demonstrates:
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:raw-html:`<pre class='highlight' style='line-height:initial;'><span>void func(const char *str) </span><span style='background-color:#4A789C'>{ </span> <span class='c1'>// Code Region from 1:28 to 6:2 with file id 0</span>
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<span style='background-color:#4A789C'> #define PUT </span><span style='background-color:#85C1F5'>printf("%s\n", str)</span><span style='background-color:#4A789C'> </span> <span class='c1'>// 2 Code Regions from 2:15 to 2:34 with file ids 1 and 2</span>
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<span style='background-color:#4A789C'> if(*str) </span>
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<span style='background-color:#4A789C'> </span><span style='background-color:#F6D55D'>PUT</span><span style='background-color:#4A789C'>; </span> <span class='c1'>// Expansion Region from 4:5 to 4:8 with file id 0 that expands a macro with file id 1</span>
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<span style='background-color:#4A789C'> </span><span style='background-color:#F6D55D'>PUT</span><span style='background-color:#4A789C'>; </span> <span class='c1'>// Expansion Region from 5:3 to 5:6 with file id 0 that expands a macro with file id 2</span>
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<span style='background-color:#4A789C'>}</span>
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</pre>`
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.. _coverage mapping counter:
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.. _coverage mapping counters:
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Counter:
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^^^^^^^^
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A coverage mapping counter can represents a reference to the profile
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instrumentation counter. The execution count for a region with such counter
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is determined by looking up the value of the corresponding profile
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instrumentation counter.
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It can also represent a binary arithmetical expression that operates on
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coverage mapping counters or other expressions.
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The execution count for a region with an expression counter is determined by
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evaluating the expression's arguments and then adding them together or
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subtracting them from one another.
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In the example below, a subtraction expression is used to compute the execution
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count for the compound statement that follows the *else* keyword:
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:raw-html:`<pre class='highlight' style='line-height:initial;'><span>int main(int argc, const char *argv[]) </span><span style='background-color:#4A789C'>{ </span> <span class='c1'>// Region's counter is a reference to the profile counter #0</span>
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<span style='background-color:#4A789C'> </span>
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<span style='background-color:#4A789C'> if (argc > 1) </span><span style='background-color:#85C1F5'>{ </span> <span class='c1'>// Region's counter is a reference to the profile counter #1</span>
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<span style='background-color:#85C1F5'> printf("%s\n", argv[1]); </span><span> </span>
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<span style='background-color:#85C1F5'> }</span><span style='background-color:#4A789C'> else </span><span style='background-color:#F6D55D'>{ </span> <span class='c1'>// Region's counter is an expression (reference to the profile counter #0 - reference to the profile counter #1)</span>
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<span style='background-color:#F6D55D'> printf("\n"); </span>
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<span style='background-color:#F6D55D'> }</span><span style='background-color:#4A789C'> </span>
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<span style='background-color:#4A789C'> return 0; </span>
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<span style='background-color:#4A789C'>}</span>
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</pre>`
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Finally, a coverage mapping counter can also represent an execution count of
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of zero. The zero counter is used to provide coverage mapping for
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unreachable statements and expressions, like in the example below:
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:raw-html:`<pre class='highlight' style='line-height:initial;'><span>int main() </span><span style='background-color:#4A789C'>{ </span>
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<span style='background-color:#4A789C'> return 0; </span>
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<span style='background-color:#4A789C'> </span><span style='background-color:#85C1F5'>printf("Hello world!\n")</span><span style='background-color:#4A789C'>; </span> <span class='c1'>// Unreachable region's counter is zero</span>
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<span style='background-color:#4A789C'>}</span>
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</pre>`
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The zero counters allow the code coverage tool to display proper line execution
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counts for the unreachable lines and highlight the unreachable code.
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Without them, the tool would think that those lines and regions were still
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executed, as it doesn't possess the frontend's knowledge.
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LLVM IR Representation
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======================
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The coverage mapping data is stored in the LLVM IR using a global constant
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structure variable called *__llvm_coverage_mapping* with the *IPSK_covmap*
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section specifier (i.e. ".lcovmap$M" on Windows and "__llvm_covmap" elsewhere).
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For example, let’s consider a C file and how it gets compiled to LLVM:
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.. _coverage mapping sample:
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.. code-block:: c
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int foo() {
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return 42;
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}
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int bar() {
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return 13;
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}
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The coverage mapping variable generated by Clang has 2 fields:
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* Coverage mapping header.
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* An optionally compressed list of filenames present in the translation unit.
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The variable has 8-byte alignment because ld64 cannot always pack symbols from
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different object files tightly (the word-level alignment assumption is baked in
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too deeply).
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.. code-block:: llvm
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@__llvm_coverage_mapping = internal constant { { i32, i32, i32, i32 }, [32 x i8] }
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{
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{ i32, i32, i32, i32 } ; Coverage map header
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{
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i32 0, ; Always 0. In prior versions, the number of affixed function records
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i32 32, ; The length of the string that contains the encoded translation unit filenames
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i32 0, ; Always 0. In prior versions, the length of the affixed string that contains the encoded coverage mapping data
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i32 3, ; Coverage mapping format version
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},
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[32 x i8] c"..." ; Encoded data (dissected later)
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}, section "__llvm_covmap", align 8
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The current version of the format is version 4. There are two differences from version 3:
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* Function records are now named symbols, and are marked *linkonce_odr*. This
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allows linkers to merge duplicate function records. Merging of duplicate
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*dummy* records (emitted for functions included-but-not-used in a translation
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unit) reduces size bloat in the coverage mapping data. As part of this
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change, region mapping information for a function is now included within the
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function record, instead of being affixed to the coverage header.
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* The filename list for a translation unit may optionally be zlib-compressed.
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The only difference between versions 3 and 2 is that a special encoding for
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column end locations was introduced to indicate gap regions.
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In version 1, the function record for *foo* was defined as follows:
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.. code-block:: llvm
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{ i8*, i32, i32, i64 } { i8* getelementptr inbounds ([3 x i8]* @__profn_foo, i32 0, i32 0), ; Function's name
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i32 3, ; Function's name length
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i32 9, ; Function's encoded coverage mapping data string length
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i64 0 ; Function's structural hash
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}
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In version 2, the function record for *foo* was defined as follows:
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.. code-block:: llvm
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{ i64, i32, i64 } {
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i64 0x5cf8c24cdb18bdac, ; Function's name MD5
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i32 9, ; Function's encoded coverage mapping data string length
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i64 0 ; Function's structural hash
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Coverage Mapping Header:
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------------------------
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The coverage mapping header has the following fields:
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* The number of function records affixed to the coverage header. Always 0, but present for backwards compatibility.
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* The length of the string in the third field of *__llvm_coverage_mapping* that contains the encoded translation unit filenames.
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* The length of the string in the third field of *__llvm_coverage_mapping* that contains any encoded coverage mapping data affixed to the coverage header. Always 0, but present for backwards compatibility.
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* The format version. The current version is 4 (encoded as a 3).
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.. _function records:
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Function record:
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----------------
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A function record is a structure of the following type:
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.. code-block:: llvm
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{ i64, i32, i64, i64, [? x i8] }
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It contains the function name's MD5, the length of the encoded mapping data for
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that function, the function's structural hash value, the hash of the filenames
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in the function's translation unit, and the encoded mapping data.
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Dissecting the sample:
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^^^^^^^^^^^^^^^^^^^^^^
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Here's an overview of the encoded data that was stored in the
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IR for the `coverage mapping sample`_ that was shown earlier:
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* The IR contains the following string constant that represents the encoded
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coverage mapping data for the sample translation unit:
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.. code-block:: llvm
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c"\01\15\1Dx\DA\13\D1\0F-N-*\D6/+\CE\D6/\C9-\D0O\CB\CF\D7K\06\00N+\07]"
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* The string contains values that are encoded in the LEB128 format, which is
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used throughout for storing integers. It also contains a compressed payload.
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* The first three LEB128-encoded numbers in the sample specify the number of
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filenames, the length of the uncompressed filenames, and the length of the
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compressed payload (or 0 if compression is disabled). In this sample, there
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is 1 filename that is 21 bytes in length (uncompressed), and stored in 29
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bytes (compressed).
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* The coverage mapping from the first function record is encoded in this string:
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.. code-block:: llvm
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c"\01\00\00\01\01\01\0C\02\02"
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This string consists of the following bytes:
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+----------+-------------------------------------------------------------------------------------------------------------------------+
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| ``0x01`` | The number of file ids used by this function. There is only one file id used by the mapping data in this function. |
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+----------+-------------------------------------------------------------------------------------------------------------------------+
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| ``0x00`` | An index into the filenames array which corresponds to the file "/Users/alex/test.c". |
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+----------+-------------------------------------------------------------------------------------------------------------------------+
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| ``0x00`` | The number of counter expressions used by this function. This function doesn't use any expressions. |
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+----------+-------------------------------------------------------------------------------------------------------------------------+
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| ``0x01`` | The number of mapping regions that are stored in an array for the function's file id #0. |
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+----------+-------------------------------------------------------------------------------------------------------------------------+
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| ``0x01`` | The coverage mapping counter for the first region in this function. The value of 1 tells us that it's a coverage |
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| | mapping counter that is a reference to the profile instrumentation counter with an index of 0. |
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+----------+-------------------------------------------------------------------------------------------------------------------------+
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| ``0x01`` | The starting line of the first mapping region in this function. |
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+----------+-------------------------------------------------------------------------------------------------------------------------+
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| ``0x0C`` | The starting column of the first mapping region in this function. |
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+----------+-------------------------------------------------------------------------------------------------------------------------+
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| ``0x02`` | The ending line of the first mapping region in this function. |
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+----------+-------------------------------------------------------------------------------------------------------------------------+
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| ``0x02`` | The ending column of the first mapping region in this function. |
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+----------+-------------------------------------------------------------------------------------------------------------------------+
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* The length of the substring that contains the encoded coverage mapping data
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for the second function record is also 9. It's structured like the mapping data
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for the first function record.
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* The two trailing bytes are zeroes and are used to pad the coverage mapping
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data to give it the 8 byte alignment.
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Encoding
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========
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The per-function coverage mapping data is encoded as a stream of bytes,
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with a simple structure. The structure consists of the encoding
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`types <cvmtypes_>`_ like variable-length unsigned integers, that
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are used to encode `File ID Mapping`_, `Counter Expressions`_ and
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the `Mapping Regions`_.
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The format of the structure follows:
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``[file id mapping, counter expressions, mapping regions]``
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The translation unit filenames are encoded using the same encoding
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`types <cvmtypes_>`_ as the per-function coverage mapping data, with the
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following structure:
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``[numFilenames : LEB128, filename0 : string, filename1 : string, ...]``
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.. _cvmtypes:
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Types
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-----
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This section describes the basic types that are used by the encoding format
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and can appear after ``:`` in the ``[foo : type]`` description.
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.. _LEB128:
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||
|
||
LEB128
|
||
^^^^^^
|
||
|
||
LEB128 is an unsigned integer value that is encoded using DWARF's LEB128
|
||
encoding, optimizing for the case where values are small
|
||
(1 byte for values less than 128).
|
||
|
||
.. _CoverageStrings:
|
||
|
||
Strings
|
||
^^^^^^^
|
||
|
||
``[length : LEB128, characters...]``
|
||
|
||
String values are encoded with a `LEB value <LEB128_>`_ for the length
|
||
of the string and a sequence of bytes for its characters.
|
||
|
||
.. _file id mapping:
|
||
|
||
File ID Mapping
|
||
---------------
|
||
|
||
``[numIndices : LEB128, filenameIndex0 : LEB128, filenameIndex1 : LEB128, ...]``
|
||
|
||
File id mapping in a function's coverage mapping stream
|
||
contains the indices into the translation unit's filenames array.
|
||
|
||
Counter
|
||
-------
|
||
|
||
``[value : LEB128]``
|
||
|
||
A `coverage mapping counter`_ is stored in a single `LEB value <LEB128_>`_.
|
||
It is composed of two things --- the `tag <counter-tag_>`_
|
||
which is stored in the lowest 2 bits, and the `counter data`_ which is stored
|
||
in the remaining bits.
|
||
|
||
.. _counter-tag:
|
||
|
||
Tag:
|
||
^^^^
|
||
|
||
The counter's tag encodes the counter's kind
|
||
and, if the counter is an expression, the expression's kind.
|
||
The possible tag values are:
|
||
|
||
* 0 - The counter is zero.
|
||
|
||
* 1 - The counter is a reference to the profile instrumentation counter.
|
||
|
||
* 2 - The counter is a subtraction expression.
|
||
|
||
* 3 - The counter is an addition expression.
|
||
|
||
.. _counter data:
|
||
|
||
Data:
|
||
^^^^^
|
||
|
||
The counter's data is interpreted in the following manner:
|
||
|
||
* When the counter is a reference to the profile instrumentation counter,
|
||
then the counter's data is the id of the profile counter.
|
||
* When the counter is an expression, then the counter's data
|
||
is the index into the array of counter expressions.
|
||
|
||
.. _Counter Expressions:
|
||
|
||
Counter Expressions
|
||
-------------------
|
||
|
||
``[numExpressions : LEB128, expr0LHS : LEB128, expr0RHS : LEB128, expr1LHS : LEB128, expr1RHS : LEB128, ...]``
|
||
|
||
Counter expressions consist of two counters as they
|
||
represent binary arithmetic operations.
|
||
The expression's kind is determined from the `tag <counter-tag_>`_ of the
|
||
counter that references this expression.
|
||
|
||
.. _Mapping Regions:
|
||
|
||
Mapping Regions
|
||
---------------
|
||
|
||
``[numRegionArrays : LEB128, regionsForFile0, regionsForFile1, ...]``
|
||
|
||
The mapping regions are stored in an array of sub-arrays where every
|
||
region in a particular sub-array has the same file id.
|
||
|
||
The file id for a sub-array of regions is the index of that
|
||
sub-array in the main array e.g. The first sub-array will have the file id
|
||
of 0.
|
||
|
||
Sub-Array of Regions
|
||
^^^^^^^^^^^^^^^^^^^^
|
||
|
||
``[numRegions : LEB128, region0, region1, ...]``
|
||
|
||
The mapping regions for a specific file id are stored in an array that is
|
||
sorted in an ascending order by the region's starting location.
|
||
|
||
Mapping Region
|
||
^^^^^^^^^^^^^^
|
||
|
||
``[header, source range]``
|
||
|
||
The mapping region record contains two sub-records ---
|
||
the `header`_, which stores the counter and/or the region's kind,
|
||
and the `source range`_ that contains the starting and ending
|
||
location of this region.
|
||
|
||
.. _header:
|
||
|
||
Header
|
||
^^^^^^
|
||
|
||
``[counter]``
|
||
|
||
or
|
||
|
||
``[pseudo-counter]``
|
||
|
||
The header encodes the region's counter and the region's kind.
|
||
|
||
The value of the counter's tag distinguishes between the counters and
|
||
pseudo-counters --- if the tag is zero, than this header contains a
|
||
pseudo-counter, otherwise this header contains an ordinary counter.
|
||
|
||
Counter:
|
||
""""""""
|
||
|
||
A mapping region whose header has a counter with a non-zero tag is
|
||
a code region.
|
||
|
||
Pseudo-Counter:
|
||
"""""""""""""""
|
||
|
||
``[value : LEB128]``
|
||
|
||
A pseudo-counter is stored in a single `LEB value <LEB128_>`_, just like
|
||
the ordinary counter. It has the following interpretation:
|
||
|
||
* bits 0-1: tag, which is always 0.
|
||
|
||
* bit 2: expansionRegionTag. If this bit is set, then this mapping region
|
||
is an expansion region.
|
||
|
||
* remaining bits: data. If this region is an expansion region, then the data
|
||
contains the expanded file id of that region.
|
||
|
||
Otherwise, the data contains the region's kind. The possible region
|
||
kind values are:
|
||
|
||
* 0 - This mapping region is a code region with a counter of zero.
|
||
* 2 - This mapping region is a skipped region.
|
||
|
||
.. _source range:
|
||
|
||
Source Range
|
||
^^^^^^^^^^^^
|
||
|
||
``[deltaLineStart : LEB128, columnStart : LEB128, numLines : LEB128, columnEnd : LEB128]``
|
||
|
||
The source range record contains the following fields:
|
||
|
||
* *deltaLineStart*: The difference between the starting line of the
|
||
current mapping region and the starting line of the previous mapping region.
|
||
|
||
If the current mapping region is the first region in the current
|
||
sub-array, then it stores the starting line of that region.
|
||
|
||
* *columnStart*: The starting column of the mapping region.
|
||
|
||
* *numLines*: The difference between the ending line and the starting line
|
||
of the current mapping region.
|
||
|
||
* *columnEnd*: The ending column of the mapping region. If the high bit is set,
|
||
the current mapping region is a gap area. A count for a gap area is only used
|
||
as the line execution count if there are no other regions on a line.
|