optimization/10258: byteswapping code fails in -O2

[warren_baird@cimmetry.com]

>Number:         10258
>Category:       optimization
>Synopsis:       byteswapping code fails in -O2
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    unassigned
>State:          open
>Class:          wrong-code
>Submitter-Id:   net
>Arrival-Date:   Fri Mar 28 22:06:00 UTC 2003
>Closed-Date:
>Last-Modified:
>Originator:     Warren Baird
>Release:        gcc-3.2.2
>Organization:
>Environment:
Solaris 8 - Sparc
>Description:
We have macros to do byteswapping of integers, and we wrote a short function to use integer macros to swap a float.  It works fine in -O0 or -O, but in -O2 it returns incorrect values.  The byte swapping macros are a bit complex, so I've attached the preprocessor output as required, but I'll include the non-preprocessed output here - it may be easier to follow...

#include <stdio.h>
#include <string.h>

typedef unsigned short  WORD,       *PWORD,    *LPWORD;
typedef unsigned long   DWORD,      *PDWORD,   *LPDWORD;
typedef float           FLOAT,      *PFLOAT;

#define BYTESWAP_LO8(s)   ((WORD)(s) & 0xff)
#define BYTESWAP_HI8(s)   (((WORD)(s) >> 8) & 0xff)
#define BYTESWAP_WORD(s)  ((WORD)((BYTESWAP_LO8(s) << 8) + (BYTESWAP_HI8(s))))

#define BYTESWAP_LO16(l)  ((DWORD)(l) & 0xffff)
#define BYTESWAP_HI16(l)  (((DWORD)(l) >> 16) & 0xffff)
#define BYTESWAP_DWORD(l) ((DWORD)(((BYTESWAP_WORD(BYTESWAP_LO16(l)) << 16) & 0xffff0000) + \
                                  (((BYTESWAP_WORD(BYTESWAP_HI16(l))) & 0x0000ffff))))

FLOAT swapFLOAT(FLOAT f)
{
	FLOAT ret;
	DWORD* pDW = (DWORD*)&ret;
	*pDW = BYTESWAP_DWORD(*(DWORD*)&f);
	return ret;
}

int main() {

	long foo = 0xfed80000;
	FLOAT f1 = 1.0;
	FLOAT sf1 = swapFLOAT(f1);

	printf("0x%08X 0x%08X\n", *(long *)&f1, *(long *)&sf1);

	return 0;

}
>How-To-Repeat:
"g++ -O testfloat.cpp" gives the expected output:
0x3F800000 0x0000803F

however, "g++ -O2 testfloat.cpp" gives the output:
0x3F800000 0x00000000

In our code, it seems to actually produce either 0, or occasionally a very large negative number.

I wasn't able to repro this on an x86-linux box...
>Fix:

>Release-Note:
>Audit-Trail:
>Unformatted:
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