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Mirror of https://github.com/RPCS3/llvm-mirror
SCEV currently fails to compute loop counts for nonunit stride loops. This comes up frequently. It prevents loop optimization and forces vectorization to insert extra loop checks. For example: void foo(int n, int *x) { for (int i = 0; i < n; i += 3) { x[i] = i; x[i+1] = i+1; x[i+2] = i+2; } } We need to properly handle the case in which limit > INT_MAX-stride. In the above case: n > INT_MAX-3. In this case the loop counter will step beyond the limit and overflow at the same time. However, knowing that signed integer overlow in undefined, we can assume the loop test behavior is arbitrary after overflow. This obeys both C undefined behavior rules, and the more strict LLVM poison value rules. I'm finally fixing this in response to Hal Finkel's persistence. The most probable reason that we never optimized this before is that we were being careful to handle case where the developer expected a side-effect free infinite loop relying on overflow: for (int i = 0; i < n; i += s) { ++j; } return j; If INT_MAX+1 is a multiple of s and n > INT_MAX-s, then we might expect an infinite loop. However there are plenty of ways to achieve this effect without relying on undefined behavior of signed overflow. llvm-svn: 193015 |
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autoconf | ||
bindings | ||
cmake | ||
docs | ||
examples | ||
include | ||
lib | ||
projects | ||
test | ||
tools | ||
unittests | ||
utils | ||
.arcconfig | ||
.clang-format | ||
.gitignore | ||
CMakeLists.txt | ||
CODE_OWNERS.TXT | ||
configure | ||
CREDITS.TXT | ||
LICENSE.TXT | ||
llvm.spec.in | ||
LLVMBuild.txt | ||
Makefile | ||
Makefile.common | ||
Makefile.config.in | ||
Makefile.rules | ||
README.txt |
Low Level Virtual Machine (LLVM) ================================ This directory and its subdirectories contain source code for the Low Level Virtual Machine, a toolkit for the construction of highly optimized compilers, optimizers, and runtime environments. LLVM is open source software. You may freely distribute it under the terms of the license agreement found in LICENSE.txt. Please see the documentation provided in docs/ for further assistance with LLVM, and in particular docs/GettingStarted.rst for getting started with LLVM and docs/README.txt for an overview of LLVM's documentation setup. If you're writing a package for LLVM, see docs/Packaging.rst for our suggestions.