http://llvm.org/bugs/show_bug.cgi?id=18048
Bug ID: 18048
Summary: Regression: NaN handling differs from LLVM 3.1-3.3
Product: libraries
Version: trunk
Hardware: PC
OS: Windows NT
Status: NEW
Severity: release blocker
Priority: P
Component: Common Code Generator Code
Assignee: [email protected]
Reporter: [email protected]
CC: [email protected]
Classification: Unclassified
Created attachment 11601
--> http://llvm.org/bugs/attachment.cgi?id=11601&action=edit
IR file demonstrating the problem.
The attached IR code is reduced from a D program. It can be compiled and run on
Linux/x86_64.
Compiled with llc -filetype=obj bug.ll ; gcc -o bug bug.o ; ./bug it produces
the output:
0000ffff - 0000ffff
but compiled with opt -O3 <bug.ll | llc -filetype=obj >bug.o ; gcc -o bug bug.o
; ./bug it produces the output:
0000ffff - 00007fff
The root cause seems to be that -real.nan is negativ but 2* -real.nan is
positiv (with -O3). That is really surprising.
The original D program is:
import core.stdc.stdio : printf;
ushort sign(real x)
{
ushort* pxe = cast(ushort *)&x;
return pxe[4];
}
void main()
{
real x = -real.nan;
real y = 2*x;
printf("%08x - %08x\n", sign(x), sign(y));
}
This happens only with LLVM3.4+, not with LLVM 3.1/3.2/3.3.
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