[llvm-branch-commits] [llvm] Backport [DAGCombine] Fix multi-use miscompile in load combine (#81586) (PR #81633)

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llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-selectiondag

Author: Nikita Popov (nikic)

<details>
<summary>Changes</summary>

(cherry picked from commit 25b9ed6e4964344e3710359bec4c831e5a8448b9)

---
Full diff: https://github.com/llvm/llvm-project/pull/81633.diff


2 Files Affected:

- (modified) llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp (+1-1) 
- (modified) llvm/test/CodeGen/X86/load-combine.ll (+32) 


``````````diff
diff --git a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index 98d8a6d9409f25..3135ec73a99e76 100644
--- a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -9253,7 +9253,7 @@ SDValue DAGCombiner::MatchLoadCombine(SDNode *N) {
 
   // Transfer chain users from old loads to the new load.
   for (LoadSDNode *L : Loads)
-    DAG.ReplaceAllUsesOfValueWith(SDValue(L, 1), SDValue(NewLoad.getNode(), 1));
+    DAG.makeEquivalentMemoryOrdering(L, NewLoad);
 
   if (!NeedsBswap)
     return NewLoad;
diff --git a/llvm/test/CodeGen/X86/load-combine.ll b/llvm/test/CodeGen/X86/load-combine.ll
index 7f8115dc1ce389..b5f3e789918813 100644
--- a/llvm/test/CodeGen/X86/load-combine.ll
+++ b/llvm/test/CodeGen/X86/load-combine.ll
@@ -1282,3 +1282,35 @@ define i32 @zext_load_i32_by_i8_bswap_shl_16(ptr %arg) {
   %tmp8 = or i32 %tmp7, %tmp30
   ret i32 %tmp8
 }
+
+define i32 @pr80911_vector_load_multiuse(ptr %ptr, ptr %clobber) nounwind {
+; CHECK-LABEL: pr80911_vector_load_multiuse:
+; CHECK:       # %bb.0:
+; CHECK-NEXT:    pushl %esi
+; CHECK-NEXT:    movl {{[0-9]+}}(%esp), %ecx
+; CHECK-NEXT:    movl {{[0-9]+}}(%esp), %edx
+; CHECK-NEXT:    movl (%edx), %esi
+; CHECK-NEXT:    movzwl (%edx), %eax
+; CHECK-NEXT:    movl $0, (%ecx)
+; CHECK-NEXT:    movl %esi, (%edx)
+; CHECK-NEXT:    popl %esi
+; CHECK-NEXT:    retl
+;
+; CHECK64-LABEL: pr80911_vector_load_multiuse:
+; CHECK64:       # %bb.0:
+; CHECK64-NEXT:    movl (%rdi), %ecx
+; CHECK64-NEXT:    movzwl (%rdi), %eax
+; CHECK64-NEXT:    movl $0, (%rsi)
+; CHECK64-NEXT:    movl %ecx, (%rdi)
+; CHECK64-NEXT:    retq
+  %load = load <4 x i8>, ptr %ptr, align 16
+  store i32 0, ptr %clobber
+  store <4 x i8> %load, ptr %ptr, align 16
+  %e1 = extractelement <4 x i8> %load, i64 1
+  %e1.ext = zext i8 %e1 to i32
+  %e1.ext.shift = shl nuw nsw i32 %e1.ext, 8
+  %e0 = extractelement <4 x i8> %load, i64 0
+  %e0.ext = zext i8 %e0 to i32
+  %res = or i32 %e1.ext.shift, %e0.ext
+  ret i32 %res
+}

``````````

</details>


https://github.com/llvm/llvm-project/pull/81633