[llvm-bugs] [Bug 24561] New: [msan] False positive on icmp sgt (trunc %x), -1

[via llvm-bugs]

https://llvm.org/bugs/show_bug.cgi?id=24561

            Bug ID: 24561
           Summary: [msan] False positive on icmp sgt (trunc %x), -1
           Product: compiler-rt
           Version: unspecified
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: compiler-rt
          Assignee: unassignedbugs at nondot.org
          Reporter: eugeni.stepanov at gmail.com
                CC: llvm-bugs at lists.llvm.org
    Classification: Unclassified

$ cat 1.cc
#include <stdio.h>

struct A {
  bool c1 : 7;
  bool c8 : 1;
  bool c9 : 1;
  A();
};

__attribute__((noinline)) A::A() : c8(false) {}

int main() {
  A* a = new A();
  if (a->c8)
    printf("zz\n");
  return 0;
}

$ clang++ 1.cc -O2 -fsanitize=memory && ./a.out
WARNING: MemorySanitizer: use-of-uninitialized-value
    #0 0x7f8dd5a8712c in main 1.cc:15:7
    ...


Bitcode for the "if" instruction:
  %10 = bitcast i8* %call to i16*
  %bf.load = load i16, i16* %10, align 1
  %11 = trunc i16 %bf.load to i8
  %bf.cast = icmp sgt i8 %11, -1
  br i1 %bf.cast, label %if.end, label %if.then

MSan instrumentation:
  %bf.load = load i16, i16* %1, align 1
  %4 = ptrtoint i16* %1 to i64
  %5 = and i64 %4, -70368744177665
  %6 = inttoptr i64 %5 to i16*
  %_msld = load i16, i16* %6, align 1

  %_msprop = trunc i16 %_msld to i8
  %7 = trunc i16 %bf.load to i8
>>> BUG
  %8 = trunc i8 %_msprop to i1
  %bf.cast = icmp sgt i8 %7, -1
<<<
  %_mscmp2 = icmp ne i1 %8, false
  br i1 %_mscmp2, label %9, label %10, !prof !1

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