https://bugs.llvm.org/show_bug.cgi?id=50569
Bug ID: 50569
Summary: Result of __builtin_ctzs() and __tzcnt_u16() not
matching after 2af95a52
Product: new-bugs
Version: trunk
Hardware: PC
OS: All
Status: NEW
Severity: normal
Priority: P
Component: new bugs
Assignee: unassignedb...@nondot.org
Reporter: douglas_y...@playstation.sony.com
CC: htmldevelo...@gmail.com, llvm-bugs@lists.llvm.org
We have an internal test that attempts to verify intrinsics are working
correctly, and the test for __tzcnt_u16 started to fail after upstream commit
2af95a527580333940333c532679356d99b88e3d. I have extracted the following repro
which should demonstrate the issue when compiled with "-mavx -mbmi -O3":
/* test.cpp */
#include <x86intrin.h>
#include <stdio.h>
typedef union {
double d64;
short i16;
} _data2;
int main(int argc, char *argv[]) {
_data2 v1, v2;
v1.d64 = v2.d64 = (short) rand();
printf("Original: v1.d64 = %lf\n", v1.d64);
printf("Original: v1.i16 = %d\n", (short) v1.d64);
v1.i16 = __builtin_ctzs( v1.i16 );
printf("__builtin_ctzs( v1.i16 ): %lf\n", v1.d64);
v2.i16 = __tzcnt_u16( v2.i16 );
printf("__tzcnt_u16( v2.i16 ): %lf\n", v2.d64);
if( v1.d64 != v2.d64 ) {
printf("Difference detected: v1.d64 = %lf, v2.d64 = %lf\n", v1.d64,
v2.d64);
printf("Difference detected: v1.i16 = 0x%x, v2.i16 = 0x%x\n", v1.i16,
v2.i16);
printf("Test failed\n");
return 1;
}
printf("Test passed\n");
return 0;
}
/* End test.cpp */
Here is a link comparing the generated assembly/run output for clang 12
(d28af7c65) vs. trunk (b2c8bcbab) on godbolt: https://godbolt.org/z/44T7eT3e6
When the version compiled by clang 12 is run, it produces the following output:
Original: v1.d64 = 17767.000000
Original: v1.i16 = 17767
__builtin_ctzs( v1.i16 ): 17767.000000
__tzcnt_u16( v2.i16 ): 17767.000000
Test passed
When the version compiled by clang trunk is run, it produces the following
output:
Original: v1.d64 = 17767.000000
Original: v1.i16 = 17767
__builtin_ctzs( v1.i16 ): 17767.000000
__tzcnt_u16( v2.i16 ): 17767.000000
Difference detected: v1.d64 = 17767.000000, v2.d64 = 17767.000000
Difference detected: v1.i16 = 0x20, v2.i16 = 0x10
Test failed
Bisecting this issue pointed to commit 2af95a527580333940333c532679356d99b88e3d
as the cause of this issue.
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