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nerfur
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Jul 20, 2025
Fix unnecessary conversion of C-String to StringRef in the `Cmp` lambda
inside `lookupLLVMIntrinsicByName`. This both fixes an ASAN error in the
code that happens when the `Name` StringRef passed in is not a Null
terminated StringRef, and additionally can potentially speed up the code
as well by eliminating the unnecessary computation of string length
every time a C String is converted to StringRef in this code (It seems
practically this computation is eliminated in optimized builds, but this
will avoid it in O0 builds as well).
Added a unit test that demonstrates this issue by building LLVM with
these options:
```
CMAKE_BUILD_TYPE=Debug
LLVM_USE_SANITIZER=Address
LLVM_OPTIMIZE_SANITIZED_BUILDS=OFF
```
The error reported is as follows:
```
==462665==ERROR: AddressSanitizer: heap-buffer-overflow on address 0x5030000391a2 at pc 0x56525cc30bbf bp 0x7fff9e4ccc60 sp 0x7fff9e4cc428
READ of size 19 at 0x5030000391a2 thread T0
#0 0x56525cc30bbe in strlen (upstream-llvm-second/llvm-project/build/unittests/IR/IRTests+0x713bbe) (BuildId: 0651acf1e582a4d2)
#1 0x7f8ff22ad334 in std::char_traits<char>::length(char const*) /usr/bin/../lib/gcc/x86_64-linux-gnu/13/../../../../include/c++/13/bits/char_traits.h:399:9
#2 0x7f8ff22a34a0 in llvm::StringRef::StringRef(char const*) /home/rjoshi/upstream-llvm-second/llvm-project/llvm/include/llvm/ADT/StringRef.h:96:33
llvm#3 0x7f8ff28ca184 in _ZZL25lookupLLVMIntrinsicByNameN4llvm8ArrayRefIjEENS_9StringRefES2_ENK3$_0clIjPKcEEDaT_T0_ upstream-llvm-second/llvm-project/llvm/lib/IR/Intrinsics.cpp:673:18
```
nerfur
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that referenced
this pull request
Jul 20, 2025
…lvm#148205) In the original motivating test case, [FoldList](https://github.com/llvm/llvm-project/blob/d8a2141ff98ee35cd1886f536ccc3548b012820b/llvm/lib/Target/AMDGPU/SIFoldOperands.cpp#L1764) had entries: ``` #0: UseMI: %224:sreg_32 = S_OR_B32 %219.sub0:sreg_64, %219.sub1:sreg_64, implicit-def dead $scc UseOpNo: 1 #1: UseMI: %224:sreg_32 = S_OR_B32 %219.sub0:sreg_64, %219.sub1:sreg_64, implicit-def dead $scc UseOpNo: 2 ``` After calling [updateOperand(#0)](https://github.com/llvm/llvm-project/blob/d8a2141ff98ee35cd1886f536ccc3548b012820b/llvm/lib/Target/AMDGPU/SIFoldOperands.cpp#L1773), [tryConstantFoldOp(#0.UseMI)](https://github.com/llvm/llvm-project/blob/d8a2141ff98ee35cd1886f536ccc3548b012820b/llvm/lib/Target/AMDGPU/SIFoldOperands.cpp#L1786) removed operand 1, and entry #&llvm#8203;1.UseOpNo was no longer valid, resulting in an [assert](https://github.com/llvm/llvm-project/blob/4a35214bddbb67f9597a500d48ab8c4fb25af150/llvm/include/llvm/ADT/ArrayRef.h#L452). This change defers constant folding until all operands have been updated so that UseOpNo values remain stable.
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