[InstCombine] Don't preserve context across div

We can't preserve the context across a non-speculatable instruction,
as this might introduce a trap. Alternatively, we could also
insert all the replacement instruction at the use-site, but that
would be a more intrusive change for the sake of this edge case.

Fixes #95547.

Author: nikic

llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp

@@ -291,10 +291,12 @@ static bool canEvaluateTruncated(Value *V, Type *Ty, InstCombinerImpl &IC,
     uint32_t BitWidth = Ty->getScalarSizeInBits();
     assert(BitWidth < OrigBitWidth && "Unexpected bitwidths!");
     APInt Mask = APInt::getBitsSetFrom(OrigBitWidth, BitWidth);
-    if (IC.MaskedValueIsZero(I->getOperand(0), Mask, 0, CxtI) &&
-        IC.MaskedValueIsZero(I->getOperand(1), Mask, 0, CxtI)) {
-      return canEvaluateTruncated(I->getOperand(0), Ty, IC, CxtI) &&
-             canEvaluateTruncated(I->getOperand(1), Ty, IC, CxtI);
+    // Do not preserve the original context instruction. Simplifying div/rem
+    // based on later context may introduce a trap.
+    if (IC.MaskedValueIsZero(I->getOperand(0), Mask, 0, I) &&
+        IC.MaskedValueIsZero(I->getOperand(1), Mask, 0, I)) {
+      return canEvaluateTruncated(I->getOperand(0), Ty, IC, I) &&
+             canEvaluateTruncated(I->getOperand(1), Ty, IC, I);
     }
     break;
   }

llvm/test/Transforms/InstCombine/trunc.ll

@@ -1097,15 +1097,15 @@ define <2 x i1> @trunc_nuw_xor_vector(<2 x i8> %x, <2 x i8> %y) {
   ret <2 x i1> %r
 }
 
-; FIXME: This is a miscompile.
 define void @pr95547(i32 %x) {
 ; CHECK-LABEL: @pr95547(
-; CHECK-NEXT:    [[X_TRUNC:%.*]] = trunc i32 [[X:%.*]] to i8
-; CHECK-NEXT:    [[DIV:%.*]] = udiv i8 11, [[X_TRUNC]]
+; CHECK-NEXT:    [[X_TRUNC:%.*]] = trunc i32 [[X:%.*]] to i16
+; CHECK-NEXT:    [[DIV:%.*]] = udiv i16 11, [[X_TRUNC]]
 ; CHECK-NEXT:    [[CMP:%.*]] = icmp ult i32 [[X]], 256
 ; CHECK-NEXT:    br i1 [[CMP]], label [[LOOP:%.*]], label [[EXIT:%.*]]
 ; CHECK:       loop:
-; CHECK-NEXT:    call void @use.i8(i8 [[DIV]])
+; CHECK-NEXT:    [[TRUNC:%.*]] = trunc nuw nsw i16 [[DIV]] to i8
+; CHECK-NEXT:    call void @use.i8(i8 [[TRUNC]])
 ; CHECK-NEXT:    br label [[LOOP]]
 ; CHECK:       exit:
 ; CHECK-NEXT:    ret void