[llvm-bugs] [Bug 33372] New: memcmp of 16 bytes is created for a 12-byte struct causing ASAN false positive

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=33372

            Bug ID: 33372
           Summary: memcmp of 16 bytes is created for a 12-byte struct
                    causing ASAN false positive
           Product: new-bugs
           Version: unspecified
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: new bugs
          Assignee: unassignedbugs at nondot.org
          Reporter: kcc at google.com
                CC: llvm-bugs at lists.llvm.org

trunk 305024, x86_64 linux
LLVM generates an out-of-bound read that is not present in the input code. 

struct A { int x, y, z; };
struct B { A a; /*gap*/ long b; };
B *bb;
int main() {
  A a1 = {1, 1, 2};
  B b1{a1, 6};
  bb = new B(b1);
} 


% clang++ -O1   z.cc -std=c++11 -S -o -  -emit-llvm
... 
%struct.A = type { i32, i32, i32 }
...
@_ZZ4mainE2a1 = private unnamed_addr constant %struct.A { i32 1, i32 1, i32 2
}, align 8

  tail call void @llvm.memcpy.p0i8.p0i8.i64(i8* nonnull %call, i8* bitcast
(%struct.A* @_ZZ4mainE2a1 to i8*), i64 16, i32 8, i1 false)

(with -O0, a regular 12-byte memcpy is emitted) 

This causes an ASAN false positive: 
% clang++ -g -O1   -fsanitize=address z.cc -std=c++11 && ./a.out 

==9157==ERROR: AddressSanitizer: global-buffer-overflow on address
0x0000004fb54c at pc 0x0000004a35b4 bp 0x7ffcac467ef0 sp 0x7ffcac4676a0
READ of size 16 at 0x0000004fb54c thread T0
    #0 0x4a35b3 in __asan_memcpy 
    #1 0x4e5b1f in main z.cc:7:12

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