[llvm] bae51e7 - [IR] handle oversized constant alloca counts in getAllocationSize (#204540)

[via llvm-commits]

Author: Arda Serdar Pektezol
Date: 2026-06-20T23:30:40+08:00
New Revision: bae51e7ffb767d08d684290d34515b91f2cbb2ed

URL: https://github.com/llvm/llvm-project/commit/bae51e7ffb767d08d684290d34515b91f2cbb2ed
DIFF: https://github.com/llvm/llvm-project/commit/bae51e7ffb767d08d684290d34515b91f2cbb2ed.diff

LOG: [IR] handle oversized constant alloca counts in getAllocationSize (#204540)

AllocaInst::getAllocationSize() unconditionally calls getZExtValue() for
array allocas, which asserts when the constant element count is wider
than 64 bits.

Use tryZExtValue() when reading the constant array size instead. If the
count cannot be represented in uint64_t, return std::nullopt rather than
asserting, matching the existing contract.

Fixes #203519

Added: 
    

Modified: 
    llvm/lib/IR/Instructions.cpp
    llvm/test/Transforms/InstCombine/alloca-big.ll

Removed: 
    


################################################################################
diff  --git a/llvm/lib/IR/Instructions.cpp b/llvm/lib/IR/Instructions.cpp
index 496a3ffa5d965..faa046729aff6 100644
--- a/llvm/lib/IR/Instructions.cpp
+++ b/llvm/lib/IR/Instructions.cpp
@@ -71,9 +71,12 @@ AllocaInst::getAllocationSize(const DataLayout &DL) const {
     auto *C = dyn_cast<ConstantInt>(getArraySize());
     if (!C)
       return std::nullopt;
+    std::optional<uint64_t> NumElements = C->getValue().tryZExtValue();
+    if (!NumElements)
+      return std::nullopt;
     assert(!Size.isScalable() && "Array elements cannot have a scalable size");
     auto CheckedProd =
-        checkedMulUnsigned(Size.getKnownMinValue(), C->getZExtValue());
+        checkedMulUnsigned(Size.getKnownMinValue(), *NumElements);
     if (!CheckedProd)
       return std::nullopt;
     return TypeSize::getFixed(*CheckedProd);

diff  --git a/llvm/test/Transforms/InstCombine/alloca-big.ll b/llvm/test/Transforms/InstCombine/alloca-big.ll
index 6925f1ba988dc..bd32ac8cab60f 100644
--- a/llvm/test/Transforms/InstCombine/alloca-big.ll
+++ b/llvm/test/Transforms/InstCombine/alloca-big.ll
@@ -1,5 +1,6 @@
 ; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
 ; RUN: opt < %s -passes=instcombine -S | FileCheck %s
+; RUN: opt < %s -passes=sroa -disable-output
 
 ; OSS-Fuzz: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=5223
 define void @test_bigalloc(ptr %dst) {