[msan] Fix "Add optional flag to improve instrumentation of disjoint OR (#145990)"

The "V1" and "V2" values were already NOT'ed, hence the calculation of
disjoint OR was incorrect. This patch fixes the issue by using the
instruction operands directly.

Author: thurstond

llvm/lib/Transforms/Instrumentation/MemorySanitizer.cpp

@@ -2512,6 +2512,7 @@ struct MemorySanitizerVisitor : public InstVisitor<MemorySanitizerVisitor> {
     //    S = S | (V1 & V2)
     Value *S1 = getShadow(&I, 0);
     Value *S2 = getShadow(&I, 1);
+    // Gotcha: V1 and V2 are NOT'ed here
     Value *V1 = IRB.CreateNot(I.getOperand(0));
     Value *V2 = IRB.CreateNot(I.getOperand(1));
     if (V1->getType() != S1->getType()) {
@@ -2524,7 +2525,11 @@ struct MemorySanitizerVisitor : public InstVisitor<MemorySanitizerVisitor> {
 
     Value *S = IRB.CreateOr({S1S2, V1S2, S1V2});
     if (ClPreciseDisjointOr && cast<PossiblyDisjointInst>(&I)->isDisjoint()) {
-      Value *V1V2 = IRB.CreateAnd(V1, V2);
+      // "V1" and "V2" were NOT'ed above, but we still want to reuse them
+      // because they were IntCast'ed to the same type as the shadows.
+      //
+      // (V1 & V2) == ~(~V1 | ~V2) (de Morgan)
+      Value *V1V2 = IRB.CreateNot(IRB.CreateOr(V1, V2));
       S = IRB.CreateOr({S, V1V2});
     }
 

llvm/test/Instrumentation/MemorySanitizer/or.ll

@@ -45,8 +45,9 @@ define i8 @test_disjoint_or(i8 %a, i8 %b) sanitize_memory {
 ; CHECK-IMPRECISE:      [[C:%.*]] = or disjoint i8 [[A]], [[B]]
 ; CHECK-IMPRECISE-NEXT: store i8 [[TMP11]], ptr @__msan_retval_tls, align 8
 ;
-; CHECK-PRECISE:         [[TMP10:%.*]] = and i8 [[TMP3]], [[TMP4]]
-; CHECK-PRECISE-NEXT:    [[TMP12:%.*]] = or i8 [[TMP11]], [[TMP10]]
+; CHECK-PRECISE:         [[TMP10:%.*]] = or i8 [[TMP3]], [[TMP4]]
+; CHECK-PRECISE-NEXT:    [[TMP11:%.*]] = xor i8 [[TMP10]], -1
+; CHECK-PRECISE-NEXT:    [[TMP12:%.*]] = or i8 [[TMP9]], [[TMP11]]
 ; CHECK-PRECISE-NEXT:    [[C:%.*]] = or disjoint i8 [[A]], [[B]]
 ; CHECK-PRECISE-NEXT:    store i8 [[TMP12]], ptr @__msan_retval_tls, align 8
 ;