[llvm-branch-commits] [llvm] release/23.x: backport PR210667 (PR #210717)

[Antonio Frighetto via llvm-branch-commits]

https://github.com/antoniofrighetto created https://github.com/llvm/llvm-project/pull/210717

Backport: 92539cf3a83e22dd589fa97d223da06339dc304a.

Requested by: @nikic, @antoniofrighetto.

>From 5d2832d2a6b274cf3c969815b2626596e34bf812 Mon Sep 17 00:00:00 2001
From: Antonio Frighetto <me at antoniofrighetto.com>
Date: Mon, 20 Jul 2026 14:58:51 +0200
Subject: [PATCH] [MemCpyOpt] Ensure call slot optz does not lower destination
 alloca alignment

A destination alloca's alignment could have been unconditionally
overwritten, letting a lower-aligned source incorrectly undo a
previous alignment increase. This issue has been addressed by
considering the maximum alignment between the new source target
and current destination, adhering to what the existing comment
already promises.
---
 llvm/lib/Transforms/Scalar/MemCpyOptimizer.cpp |  3 ++-
 llvm/test/Transforms/MemCpyOpt/callslot.ll     | 18 ++++++++++++++++++
 2 files changed, 20 insertions(+), 1 deletion(-)

diff --git a/llvm/lib/Transforms/Scalar/MemCpyOptimizer.cpp b/llvm/lib/Transforms/Scalar/MemCpyOptimizer.cpp
index 3fc84f2fc8c13..92e1d9cf21240 100644
--- a/llvm/lib/Transforms/Scalar/MemCpyOptimizer.cpp
+++ b/llvm/lib/Transforms/Scalar/MemCpyOptimizer.cpp
@@ -1075,7 +1075,8 @@ bool MemCpyOptPass::performCallSlotOptzn(Instruction *cpyLoad,
   // If the destination wasn't sufficiently aligned then increase its alignment.
   if (!isDestSufficientlyAligned) {
     assert(isa<AllocaInst>(cpyDest) && "Can only increase alloca alignment!");
-    cast<AllocaInst>(cpyDest)->setAlignment(srcAlign);
+    AllocaInst *DestAlloca = cast<AllocaInst>(cpyDest);
+    DestAlloca->setAlignment(std::max(DestAlloca->getAlign(), srcAlign));
   }
 
   if (NeedMoveGEP) {
diff --git a/llvm/test/Transforms/MemCpyOpt/callslot.ll b/llvm/test/Transforms/MemCpyOpt/callslot.ll
index 6c999c57f9187..25698a97ea145 100644
--- a/llvm/test/Transforms/MemCpyOpt/callslot.ll
+++ b/llvm/test/Transforms/MemCpyOpt/callslot.ll
@@ -234,6 +234,24 @@ define void @dest_not_writable(ptr noalias dereferenceable(128) %dst) {
   ret void
 }
 
+; Ensure call slot optimization does not attempt to lower the existing destination
+; alloca's alignment.
+define void @dest_alignment(ptr %src) {
+; CHECK-LABEL: @dest_alignment(
+; CHECK-NEXT:    [[SRC_2:%.*]] = alloca [24 x i8], align 4
+; CHECK-NEXT:    [[DST:%.*]] = alloca [24 x i8], align 16
+; CHECK-NEXT:    call void @llvm.memcpy.p0.p0.i64(ptr align 16 [[DST]], ptr [[SRC:%.*]], i64 24, i1 false)
+; CHECK-NEXT:    call void @accept_ptr(ptr [[DST]])
+; CHECK-NEXT:    ret void
+;
+  %src.2 = alloca [24 x i8], align 4
+  %dst = alloca [24 x i8], align 16
+  call void @llvm.memcpy.p0.p0.i64(ptr align 16 %dst, ptr %src, i64 24, i1 false)
+  call void @accept_ptr(ptr %src.2)
+  call void @llvm.memcpy.p0.p0.i64(ptr %dst, ptr %src.2, i64 24, i1 false)
+  ret void
+}
+
 declare void @may_throw()
 declare void @accept_ptr(ptr)
 declare void @llvm.memcpy.p0.p0.i64(ptr, ptr, i64, i1)