[llvm] [InstCombine] Ignore frees when inferring inbounds (PR #202304)

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llvmorg-github-actions[bot] wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-transforms

Author: Nikita Popov (nikic)

<details>
<summary>Changes</summary>

>From LangRef:

> Note that the object does not have to be live anymore; being
> in-bounds of a deallocated object is sufficient.

---
Full diff: https://github.com/llvm/llvm-project/pull/202304.diff


2 Files Affected:

- (modified) llvm/lib/Transforms/InstCombine/InstructionCombining.cpp (+3-1) 
- (modified) llvm/test/Transforms/InstCombine/inbounds-gep.ll (+1) 


``````````diff
diff --git a/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp b/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
index 89bf4bf713421..608966a5e9289 100644
--- a/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
@@ -3615,7 +3615,9 @@ Instruction *InstCombinerImpl::visitGetElementPtrInst(GetElementPtrInst &GEP) {
     bool CanBeNull, CanBeFreed;
     uint64_t DerefBytes = UnderlyingPtrOp->getPointerDereferenceableBytes(
         DL, CanBeNull, CanBeFreed);
-    if (!CanBeNull && !CanBeFreed && DerefBytes != 0) {
+    // We can ignore CanBeFreed here, because inbounds is explicitly allowed to
+    // refer to a deallocated object.
+    if (!CanBeNull && DerefBytes != 0) {
       if (GEP.accumulateConstantOffset(DL, BasePtrOffset) &&
           BasePtrOffset.isNonNegative()) {
         APInt AllocSize(IdxWidth, DerefBytes);
diff --git a/llvm/test/Transforms/InstCombine/inbounds-gep.ll b/llvm/test/Transforms/InstCombine/inbounds-gep.ll
index 467e5a82f89cc..0ea71da485766 100644
--- a/llvm/test/Transforms/InstCombine/inbounds-gep.ll
+++ b/llvm/test/Transforms/InstCombine/inbounds-gep.ll
@@ -1,5 +1,6 @@
 ; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 2
 ; RUN: opt -passes=instcombine -S < %s | FileCheck %s
+; RUN: opt -passes=instcombine -use-dereferenceable-at-point-semantics -S < %s | FileCheck %s
 
 declare ptr @g()
 declare ptr @use(ptr)

``````````

</details>


https://github.com/llvm/llvm-project/pull/202304