[llvm] 25b9ed6 - [DAGCombine] Fix multi-use miscompile in load combine (#81586)

[via llvm-commits]

Author: Nikita Popov
Date: 2024-02-13T16:41:00+01:00
New Revision: 25b9ed6e4964344e3710359bec4c831e5a8448b9

URL: https://github.com/llvm/llvm-project/commit/25b9ed6e4964344e3710359bec4c831e5a8448b9
DIFF: https://github.com/llvm/llvm-project/commit/25b9ed6e4964344e3710359bec4c831e5a8448b9.diff

LOG: [DAGCombine] Fix multi-use miscompile in load combine (#81586)

The load combine replaces a number of original loads with one new loads
and also replaces the output chains of the original loads with the
output chain of the new load. This is incorrect if the original load is
retained (due to multi-use), as it may get incorrectly reordered.

Fix this by using makeEquivalentMemoryOrdering() instead, which will
create a TokenFactor with both chains.

Fixes https://github.com/llvm/llvm-project/issues/80911.

Added: 
    

Modified: 
    llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
    llvm/test/CodeGen/X86/load-combine.ll

Removed: 
    


################################################################################
diff  --git a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index d3cd9b1671e1b9..52011e593f2e0a 100644
--- a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -9252,7 +9252,7 @@ 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 7e4e11fcc75c20..b5f3e789918813 100644
--- a/llvm/test/CodeGen/X86/load-combine.ll
+++ b/llvm/test/CodeGen/X86/load-combine.ll
@@ -1283,7 +1283,6 @@ define i32 @zext_load_i32_by_i8_bswap_shl_16(ptr %arg) {
   ret i32 %tmp8
 }
 
-; FIXME: This is a miscompile.
 define i32 @pr80911_vector_load_multiuse(ptr %ptr, ptr %clobber) nounwind {
 ; CHECK-LABEL: pr80911_vector_load_multiuse:
 ; CHECK:       # %bb.0:
@@ -1299,9 +1298,9 @@ define i32 @pr80911_vector_load_multiuse(ptr %ptr, ptr %clobber) nounwind {
 ;
 ; 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 (%rdi), %ecx
 ; CHECK64-NEXT:    movl %ecx, (%rdi)
 ; CHECK64-NEXT:    retq
   %load = load <4 x i8>, ptr %ptr, align 16