Message-ID:	<35DFF50D.7F9E4F2B@gmx.net>
Date:	Sun, 23 Aug 1998 10:55:09 +0000
From:	Robert Hoehne <robert DOT hoehne AT gmx DOT net>
Organization:	none provided
MIME-Version:	1.0
To:	Eli Zaretskii <eliz AT is DOT elta DOT co DOT il>
CC:	djgpp-workers AT delorie DOT com
Subject:	Re: Strange code generated by GCC
References:	<Pine DOT SUN DOT 3 DOT 91 DOT 980818141311 DOT 12510B-100000 AT is>

Eli Zaretskii wrote :
> 
> I can solve the problem by using memcpy instead of the last assignment in
> SET_FLOAT_WORD macro, but I'd like to understand why is GCC generate such
> code, and why optimizations change that.

In my opinion this is a bug in gcc. If you look at the generated asm
code you will see, that the macros are totally optimized out and then
there is only the following for the three lines generated:

>   SET_FLOAT_WORD(fv, 0x7fc00000U);
>   GET_FLOAT_WORD(iv, fv);
>   printf ("SET_FLOAT_WORD: %f, GET_FLOAT_WORD: %x\n", fv, iv);
(output from as -ahl test.s -o test.o


  39:test.c        ****   SET_FLOAT_WORD(fv, 0x7fc00000U);
  68              	.stabn 68,0,39,LM3-_main
  69              	LM3:
  70 003a 83C408   		addl $8,%esp
  40:test.c        ****   GET_FLOAT_WORD(iv, fv);
  41:test.c        ****   printf ("SET_FLOAT_WORD: %f, GET_FLOAT_WORD:
%x\n", fv, iv);
  71              	.stabn 68,0,41,LM4-_main
  72              	LM4:
  73 003d 680000C0 		pushl $-4194304
  73      FF
  74 0042 680000F8 		pushl $-524288
  74      FF
  75 0047 6A00     		pushl $0
  76 0049 68000000 		pushl $LC0
  76      00
  77 004e E8ADFFFF 		call _printf


As you can see, the lines starting with 73 show, that gcc pushes
there already 0xffc00000 on the stack, which means it is already
wrong calculated by the optimizing internals of gcc.

If you use the trick with the memcpy in the macros, gcc will not
be able to completely optimizing out the macros and it will work
correct.

Robert
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