Juneyoung Lee a7f581c4d0 [InstCombine] use poison as placeholder for undemanded elems ... Currently undef is used as a don’t-care vector when constructing a vector using a series of insertelement. However, this is problematic because undef isn’t undefined enough. Especially, a sequence of insertelement can be optimized to shufflevector, but using undef as its placeholder makes shufflevector a poison-blocking instruction because undef cannot be optimized to poison. This makes a few straightforward optimizations incorrect, such as: ``` ; https://bugs.llvm.org/show_bug.cgi?id=44185 define <4 x float> @insert_not_undef_shuffle_translate_commute(float %x, <4 x float> %y, <4 x float> %q) { %xv = insertelement <4 x float> %q, float %x, i32 2 %r = shufflevector <4 x float> %y, <4 x float> %xv, <4 x i32> { 0, 6, 2, undef } ret <4 x float> %r ; %r[3] is undef } => define <4 x float> @insert_not_undef_shuffle_translate_commute(float %x, <4 x float> %y, <4 x float> %q) { %r = insertelement <4 x float> %y, float %x, i32 1 ret <4 x float> %r ; %r[3] = %y[3], incorrect if %y[3] = poison } Transformation doesn't verify! ERROR: Target is more poisonous than source ``` I’d like to suggest 1. Using poison as insertelement’s placeholder value (IRBuilder::CreateVectorSplat should be patched too) 2. Updating shufflevector’s semantics to return poison element if mask is undef Note that poison is currently lowered into UNDEF in SelDag, so codegen part is okay. m_Undef() matches PoisonValue as well, so existing optimizations will still fire. The only concern is hidden miscompilations that will go incorrect when poison constant is given. A conservative way is copying all tests having `insertelement undef` & replacing it with `insertelement poison` & run Alive2 on it, but it will create many tests and people won’t like it. :( Instead, I’ll simply locally maintain the tests and run Alive2. If there is any bug found, I’ll report it. Relevant links: https://bugs.llvm.org/show_bug.cgi?id=43958 , http://lists.llvm.org/pipermail/llvm-dev/2019-November/137242.html Reviewed By: nikic Differential Revision: https://reviews.llvm.org/D93586		2020-12-28 08:58:15 +09:00
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Analysis	[test] Pin some tests to legacy PM	2020-12-26 13:46:02 -08:00
Assembler	[DebugInfo] Support Fortran 'use <external module>' statement.	2020-12-18 13:10:57 -05:00
Bindings
Bitcode	[DebugInfo] Support Fortran 'use <external module>' statement.	2020-12-18 13:10:57 -05:00
BugPoint
CodeGen	[InstCombine] use poison as placeholder for undemanded elems	2020-12-28 08:58:15 +09:00
DebugInfo	Precommit analysis/etc tests for inselt poison placeholder	2020-12-24 12:14:24 +09:00
Demangle
Examples
ExecutionEngine
Feature
FileCheck	[FileCheck] Add a literal check directive modifier	2020-12-18 17:26:15 -08:00
Instrumentation
Integer
JitListener
Linker
LTO
MachineVerifier
MC	[RISCV] Improve VMConstraint checking on more unary and nullary instructions.	2020-12-26 18:47:59 -08:00
Object	[llvm-readobj] - Dump the ELF file type better.	2020-12-23 11:13:19 +03:00
ObjectYAML
Other	Precommit analysis/etc tests for inselt poison placeholder	2020-12-24 12:14:24 +09:00
Reduce	[llvm-reduce][test] Make remove-alias.ll CHECK patterns more specific after D90302	2020-12-20 20:06:02 -08:00
SafepointIRVerifier
Support
SymbolRewriter
TableGen	[TableGen] Add the !substr() bang operator	2020-12-23 10:59:33 -05:00
ThinLTO/X86
tools	[obj2yaml] - Dump the content of a broken hash table properly.	2020-12-25 11:51:28 +03:00
Transforms	[InstCombine] use poison as placeholder for undemanded elems	2020-12-28 08:58:15 +09:00
Unit
Verifier
YAMLParser
.clang-format
CMakeLists.txt
lit.cfg.py
lit.site.cfg.py.in
TestRunner.sh