[llvm] be687af - [InstSimplify] Adjust context instruction when threading phi (PR61312)

[Nikita Popov via llvm-commits]

Author: Nikita Popov
Date: 2023-03-10T10:34:27+01:00
New Revision: be687aff052bbdd6fd2cc3c75c8cebdd9ce7fab2

URL: https://github.com/llvm/llvm-project/commit/be687aff052bbdd6fd2cc3c75c8cebdd9ce7fab2
DIFF: https://github.com/llvm/llvm-project/commit/be687aff052bbdd6fd2cc3c75c8cebdd9ce7fab2.diff

LOG: [InstSimplify] Adjust context instruction when threading phi (PR61312)

When threading operations over phis, we need to adjust the context
instruction to the terminator of the incoming block. This was
handled when threading icmps, but not when threading binops.

Fixes https://github.com/llvm/llvm-project/issues/61312.

Added: 
    

Modified: 
    llvm/lib/Analysis/InstructionSimplify.cpp
    llvm/test/Transforms/InstSimplify/phi.ll

Removed: 
    


################################################################################
diff  --git a/llvm/lib/Analysis/InstructionSimplify.cpp b/llvm/lib/Analysis/InstructionSimplify.cpp
index abcde1ded6b2c..c57a111a5e204 100644
--- a/llvm/lib/Analysis/InstructionSimplify.cpp
+++ b/llvm/lib/Analysis/InstructionSimplify.cpp
@@ -539,12 +539,16 @@ static Value *threadBinOpOverPHI(Instruction::BinaryOps Opcode, Value *LHS,
 
   // Evaluate the BinOp on the incoming phi values.
   Value *CommonValue = nullptr;
-  for (Value *Incoming : PI->incoming_values()) {
+  for (Use &Incoming : PI->incoming_values()) {
     // If the incoming value is the phi node itself, it can safely be skipped.
     if (Incoming == PI)
       continue;
-    Value *V = PI == LHS ? simplifyBinOp(Opcode, Incoming, RHS, Q, MaxRecurse)
-                         : simplifyBinOp(Opcode, LHS, Incoming, Q, MaxRecurse);
+    Instruction *InTI = PI->getIncomingBlock(Incoming)->getTerminator();
+    Value *V = PI == LHS
+                   ? simplifyBinOp(Opcode, Incoming, RHS,
+                                   Q.getWithInstruction(InTI), MaxRecurse)
+                   : simplifyBinOp(Opcode, LHS, Incoming,
+                                   Q.getWithInstruction(InTI), MaxRecurse);
     // If the operation failed to simplify, or simplified to a 
diff erent value
     // to previously, then give up.
     if (!V || (CommonValue && V != CommonValue))

diff  --git a/llvm/test/Transforms/InstSimplify/phi.ll b/llvm/test/Transforms/InstSimplify/phi.ll
index 14f17674bc524..c97326ea05137 100644
--- a/llvm/test/Transforms/InstSimplify/phi.ll
+++ b/llvm/test/Transforms/InstSimplify/phi.ll
@@ -153,7 +153,8 @@ EXIT:
   ret i8 %r
 }
 
-; FIXME: This is a miscompile.
+; Should not fold srem to -1 due to incorrect context instruction when
+; threading over phi.
 define i32 @pr61312() {
 ; CHECK-LABEL: @pr61312(
 ; CHECK-NEXT:  entry:
@@ -166,7 +167,8 @@ define i32 @pr61312() {
 ; CHECK-NEXT:    [[DEC]] = add nsw i32 [[A_0]], -1
 ; CHECK-NEXT:    br label [[FOR_COND]]
 ; CHECK:       for.end:
-; CHECK-NEXT:    ret i32 -1
+; CHECK-NEXT:    [[REM:%.*]] = srem i32 -1, [[A_0]]
+; CHECK-NEXT:    ret i32 [[REM]]
 ;
 entry:
   br label %for.cond