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[ale] gcc optimization problem
- Subject: [ale] gcc optimization problem
- From: astewart at layton-graphics.com (D. Alan Stewart)
- Date: Wed, 6 Mar 2002 08:06:38 -0500
The problem started when I switched to a new machine for compiling. I don't
have ready access to the old machine so I don't know what version of gcc
was on it, but most certainly it was older. The code is believed to be good,
I've been using it for quite some time (over a year) with no observed bugs.
However, the specific case of 0.0 is very unlikely to occur in my input data.
Thanks, I'll check on that.
For now I've just turned optimizations off and everything works. I thought
maybe someone might know enough about gcc's optimizations to suggest
what I need to change to allow the compiler to handle this function correctly.
I suppose there's probaby a pragma I can use to tell the compile to not
optimize this particular function.
> For what it's worth, -O2 compilation of your code worked for me, but
> I used gcc version 2.95.3. The disassembled object code (included
> below) looks to do what it should. I presume gcc-2.96.98 is
> generating different code for you.
> Incidentally, shouldn't your function handle the special case of 0.0
> on input? I believe it presently translates 0.0 to 1.5e-39
> (= 2^-129).
> readFaxFloat.o: file format elf32-i386
> Disassembly of section .text:
> 00000000 <readVaxFloat>:
> 0: 55 push %ebp
> 1: 89 e5 mov %esp,%ebp
> 3: 83 ec 10 sub $0x10,%esp
> 6: 56 push %esi
> 7: 53 push %ebx
> 8: 8b 5d 08 mov 0x8(%ebp),%ebx
> b: 0f b7 0b movzwl (%ebx),%ecx
> e: 89 c8 mov %ecx,%eax
> 10: 25 80 7f 00 00 and $0x7f80,%eax
> 15: 89 ca mov %ecx,%edx
> 17: 66 c1 e8 03 shr $0x3,%ax
> 1b: 05 e0 37 00 00 add $0x37e0,%eax
> 20: 81 e2 00 80 ff ff and $0xffff8000,%edx
> 26: 09 c2 or %eax,%edx
> 28: 89 c8 mov %ecx,%eax
> 2a: 83 e0 7f and $0x7f,%eax
> 2d: 8d 75 f8 lea 0xfffffff8(%ebp),%esi
> 30: 66 c1 e8 03 shr $0x3,%ax
> 34: 09 c2 or %eax,%edx
> 36: 66 89 56 06 mov %dx,0x6(%esi)
> 3a: 0f b7 53 02 movzwl 0x2(%ebx),%edx
> 3e: 89 d0 mov %edx,%eax
> 40: 66 c1 e8 03 shr $0x3,%ax
> 44: c1 e1 0d shl $0xd,%ecx
> 47: 09 c1 or %eax,%ecx
> 49: 66 89 4e 04 mov %cx,0x4(%esi)
> 4d: 0f b7 4b 04 movzwl 0x4(%ebx),%ecx
> 51: 89 c8 mov %ecx,%eax
> 53: 66 c1 e8 03 shr $0x3,%ax
> 57: c1 e2 0d shl $0xd,%edx
> 5a: 09 c2 or %eax,%edx
> 5c: 66 89 56 02 mov %dx,0x2(%esi)
> 60: 0f b7 43 06 movzwl 0x6(%ebx),%eax
> 64: c1 e1 0d shl $0xd,%ecx
> 67: 66 c1 e8 03 shr $0x3,%ax
> 6b: 09 c1 or %eax,%ecx
> 6d: 66 89 4d f8 mov %cx,0xfffffff8(%ebp)
> 71: 5b pop %ebx
> 72: dd 45 f8 fldl 0xfffffff8(%ebp)
> 75: 5e pop %esi
> 76: c9 leave
> 77: c3 ret
D. Alan Stewart
Layton Graphics, Inc.
155 Woolco Dr.
Marietta, GA 30062
"As far as the laws of mathematics refer to reality, they
are not certain; and as far as they are certain, they do
not refer to reality." - Albert Einstein
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