https://bugs.llvm.org/show_bug.cgi?id=37523
Bug ID: 37523
Summary: MemorySanitizer false positive in divide-by-undef
Product: libraries
Version: trunk
Hardware: PC
OS: Linux
Status: NEW
Severity: enhancement
Priority: P
Component: Miscellaneous Instrumentation passes
Assignee: unassignedb...@nondot.org
Reporter: eugeni.stepa...@gmail.com
CC: llvm-bugs@lists.llvm.org
#include <xmmintrin.h>
#include <sanitizer/msan_interface.h>
int main() {
volatile int scale = 5;
auto zz = _mm_div_ps(_mm_set1_ps(255), _mm_set1_ps(scale));
__msan_print_shadow(&zz, sizeof(zz));
}
Until recently, it was represented like this in the IR:
%vecinit.i = insertelement <4 x float> undef, float %conv, i32 0
%vecinit3.i = shufflevector <4 x float> %vecinit.i, <4 x float> undef, <4 x
i32> zeroinitializer
%div.i = fdiv <4 x float> <float 2.550000e+02, float 2.550000e+02, float
2.550000e+02, float 2.550000e+02>, %vecinit3.i
The something changed, and now we fill the first lane only, divide, and then
broadcast:
%vecinit.i = insertelement <4 x float> undef, float %conv, i32 0
%0 = fdiv <4 x float> <float 2.550000e+02, float undef, float undef, float
undef>, %vecinit.i
%1 = shufflevector <4 x float> %0, <4 x float> undef, <4 x i32>
zeroinitializer
This in effect divides undef by undef and throws away the result.
MSan instruments division with a strict check for the divisor's shadow,
assuming that division by an uninitialized value may trap. This is not aligned
with IR semantics: LangRef describes fdiv as side effect free.
MSan should delay the check until the value is actually used (or not, like in
this case) by combining and propagating shadow.
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