http://llvm.org/bugs/show_bug.cgi?id=3851
Summary: misscompilation : INSTCOMBINE ITERATION generating undef
ashr
Product: libraries
Version: trunk
Platform: PC
OS/Version: Linux
Status: NEW
Severity: normal
Priority: P2
Component: Global Analyses
AssignedTo: [email protected]
ReportedBy: [email protected]
CC: [email protected]
$ cat mm2.c
#include <stdint.h>
#define FILTER_SHIFT 15
int foo(int32_t val)
{
short dst;
val = (val + (1<<(FILTER_SHIFT-1)))>>FILTER_SHIFT;
dst = (unsigned)(val + 32768) > 65535 ? (val>>31) ^ 32767 : val;
printf("%x\n", dst);
}
int main()
{
foo(0xb6a00000);
}
Building with clang,
at -O0 it generate ffff8000
at -O3 it generate 7fff
It seems to be because of :
INSTCOMBINE ITERATION #0 on foo
IC: Old = %shr4 = ashr i32 %shr, 31 ; <i32> [#uses=1]
New = <badref> = ashr i32 %add, 32 ; <i32> [#uses=0]
[...]
IC: DCE: %shr48 = lshr i32 undef, 32 ; <i32> [#uses=0]
IC: DCE: %shr4 = ashr i32 %add, 32 ; <i32> [#uses=0]
where does the "ashr i32 %add, 32" comes from ?
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