[Clang][CodeGen] Preserve alignment information for pointer arithmetics (#152575)

Previously, the alignment of pointer arithmetics was inferred from the
pointee type, losing the alignment information from its operands:

https://github.com/llvm/llvm-project/blob/503c0908c3450d228debd64baecf41df8f58476e/clang/lib/CodeGen/CGExpr.cpp#L1446-L1449

This patch preserves alignment information for pointer arithmetics `P
+/- C`, to match the behavior of identical array subscript `&P[C]`:
https://godbolt.org/z/xx1hfTrx4.

Closes https://github.com/llvm/llvm-project/issues/152330. Although the
motivating case can be fixed by
https://github.com/llvm/llvm-project/pull/145733, the alignment cannot
be recovered without a dominating memory access with larger alignment.

1 files changed, 3 insertions, 1 deletions

diff --git a/clang/lib/CodeGen/CGExprScalar.cpp b/clang/lib/CodeGen/CGExprScalar.cpp
index fcbfa5e31d14..1e23f4554674 100644
--- a/clang/lib/CodeGen/CGExprScalar.cpp
+++ b/clang/lib/CodeGen/CGExprScalar.cpp
@@ -4186,7 +4186,9 @@ Value *ScalarExprEmitter::EmitOverflowCheckedBinOp(const BinOpInfo &Ops) {
   return phi;
 }
 
-/// This function is used for BO_Add/BO_Sub/BO_AddAssign/BO_SubAssign.
+/// BO_Add/BO_Sub are handled by EmitPointerWithAlignment to preserve alignment
+/// information.
+/// This function is used for BO_AddAssign/BO_SubAssign.
 static Value *emitPointerArithmetic(CodeGenFunction &CGF, const BinOpInfo &op,
                                     bool isSubtraction) {
   // Must have binary (not unary) expr here.  Unary pointer