https://github.com/Lokesh-Reddy011 created
https://github.com/llvm/llvm-project/pull/167016
### Description
This patch fixes an issue where constexpr evaluation failed for bitcasts
between vector and integer types of equal bit width (e.g., `__m64` ↔ `long
long`).
The change updates `ExprConstant.cpp` to allow reinterpretation when source and
destination types have matching bit sizes. In that case, the `APValue` is
copied directly instead of producing an error.
#### Example
The following code previously failed but now compiles successfully with the
patch:
```cpp
#include <immintrin.h>
constexpr __m64 foo(__m64 a, int b) {
return (__m64)(long long)a << b;
}
int main() {
foo((__m64){5}, 1);
return 0;
}
```
#### Testing
* Verified using `clang++ -std=c++20` — works correctly after the patch.
* Test not added to `clang/test` since it requires `<immintrin.h>`, whose path
varies across systems.
>From 9e1edbf9f2317889c494009a17735edafc7a003e Mon Sep 17 00:00:00 2001
From: Lokesh <[email protected]>
Date: Sat, 8 Nov 2025 01:44:51 +0530
Subject: [PATCH] [clang] Support constexpr bitcasts between equal-size vector
and integer types (#159905)
---
clang/lib/AST/ExprConstant.cpp | 19 ++++++++++++++++++-
1 file changed, 18 insertions(+), 1 deletion(-)
diff --git a/clang/lib/AST/ExprConstant.cpp b/clang/lib/AST/ExprConstant.cpp
index 97eeba8b9d6cc..5b8900f02aeb9 100644
--- a/clang/lib/AST/ExprConstant.cpp
+++ b/clang/lib/AST/ExprConstant.cpp
@@ -17078,7 +17078,24 @@ bool IntExprEvaluator::VisitCastExpr(const CastExpr
*E) {
case CK_HLSLAggregateSplatCast:
llvm_unreachable("invalid cast kind for integral value");
- case CK_BitCast:
+ case CK_BitCast:{
+ APValue Sub;
+ if (!Evaluate(Sub, Info, E->getSubExpr()))
+ return false;
+
+ QualType SrcTy = E->getSubExpr()->getType();
+ QualType DstTy = E->getType();
+
+ // Allow reinterpretation if bit widths match
+ if (Info.Ctx.getTypeSize(SrcTy) == Info.Ctx.getTypeSize(DstTy)) {
+ // Use APValue::BitCast if available, else just copy value bits
+ Result = Sub;
+ return true;
+ }
+
+ return Error(E);
+}
+
case CK_Dependent:
case CK_LValueBitCast:
case CK_ARCProduceObject:
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