forked from OSchip/llvm-project
7a67ed5795
Summary:
This is again motivated by D67122 sanitizer check enhancement.
That patch seemingly worsens `-fsanitize=pointer-overflow`
overhead from 25% to 50%, which strongly implies missing folds.
In this particular case, given
```
char* test(char& base, unsigned long offset) {
return &base - offset;
}
```
it will end up producing something like
https://godbolt.org/z/luGEju
which after optimizations reduces down to roughly
```
declare void @use64(i64)
define i1 @test(i8* dereferenceable(1) %base, i64 %offset) {
%base_int = ptrtoint i8* %base to i64
%adjusted = sub i64 %base_int, %offset
call void @use64(i64 %adjusted)
%not_null = icmp ne i64 %adjusted, 0
%no_underflow = icmp ule i64 %adjusted, %base_int
%no_underflow_and_not_null = and i1 %not_null, %no_underflow
ret i1 %no_underflow_and_not_null
}
```
Without D67122 there was no `%not_null`,
and in this particular case we can "get rid of it", by merging two checks:
Here we are checking: `Base u>= Offset && (Base u- Offset) != 0`, but that is simply `Base u> Offset`
Alive proofs:
https://rise4fun.com/Alive/QOs
The `@llvm.usub.with.overflow` pattern itself is not handled here
because this is the main pattern, that we currently consider canonical.
https://bugs.llvm.org/show_bug.cgi?id=43251
Reviewers: spatel, nikic, xbolva00, majnemer
Reviewed By: xbolva00, majnemer
Subscribers: vsk, majnemer, xbolva00, hiraditya, llvm-commits
Tags: #llvm
Differential Revision: https://reviews.llvm.org/D67356
llvm-svn: 372341
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| .. | ||
| CMakeLists.txt | ||
| InstCombineAddSub.cpp | ||
| InstCombineAndOrXor.cpp | ||
| InstCombineAtomicRMW.cpp | ||
| InstCombineCalls.cpp | ||
| InstCombineCasts.cpp | ||
| InstCombineCompares.cpp | ||
| InstCombineInternal.h | ||
| InstCombineLoadStoreAlloca.cpp | ||
| InstCombineMulDivRem.cpp | ||
| InstCombinePHI.cpp | ||
| InstCombineSelect.cpp | ||
| InstCombineShifts.cpp | ||
| InstCombineSimplifyDemanded.cpp | ||
| InstCombineTables.td | ||
| InstCombineVectorOps.cpp | ||
| InstructionCombining.cpp | ||
| LLVMBuild.txt | ||