[MSan] Handle llvm.is.constant intrinsic

[glider]

Summary

MSan used to report false positives in the case the argument of llvm.is.constant intrinsic was uninitialized.
In fact checking this argument is unnecessary, as the intrinsic is only used at compile time, and its value doesn't depend on the value of the argument.

[dvyukov]

Nice! Interesting nobody reported this false positive on user-space code.

[glider]

Landed r350173, thank you!

lib/Transforms/Instrumentation/MemorySanitizer.cpp

void visitIntrinsicInst(IntrinsicInst &I) {
     case Intrinsic::x86_sse_cmp_ps:
     case Intrinsic::x86_sse2_cmp_pd:
       // FIXME: For x86_avx_cmp_pd_256 and x86_avx_cmp_ps_256 this function
       // generates reasonably looking IR that fails in the backend with "Do not
       // know how to split the result of this operator!".
       handleVectorComparePackedIntrinsic(I);
       break;
 
+    case Intrinsic::is_constant:
+      // The result of llvm.is.constant() is always defined.
+      setShadow(&I, getCleanShadow(&I));
+      setOrigin(&I, getCleanOrigin());
+      break;
+
     default:
       if (!handleUnknownIntrinsic(I))
         visitInstruction(I);
       break;
     }
   }
 
   void visitCallSite(CallSite CS) {

test/Instrumentation/MemorySanitizer/msan_llvm_is_constant.ll

+; Make sure MSan doesn't insert shadow checks for @llvm.is.constant.* arguments.
+
+; RUN: opt < %s -msan -msan-kernel=1 -S | FileCheck -check-prefixes=CHECK %s
+; RUN: opt < %s -msan -S | FileCheck -check-prefixes=CHECK %s
+
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+; Function Attrs: nounwind readnone uwtable
+define dso_local i32 @bar(i32 %v) local_unnamed_addr sanitize_memory {
+entry:
+  %0 = tail call i1 @llvm.is.constant.i32(i32 %v)
+  %1 = zext i1 %0 to i32
+  ret i32 %1
+}
+
+; CHECK-LABEL: bar
+; CHECK-NOT: call void @__msan_warning
+
+; Function Attrs: nounwind readnone
+declare i1 @llvm.is.constant.i32(i32)