Messages in this thread | | | Date | Mon, 7 May 2007 20:39:31 +0200 | From | Johannes Stezenbach <> | Subject | Re: [patch] CFS scheduler, -v8 |
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On Mon, May 07, 2007, Linus Torvalds wrote: > On Mon, 7 May 2007, Esben Nielsen wrote: > > > > What is (long)(a-b) ? I have tried to look it up in the C99 standeard but I > > can't find it. Maybe it is in the referred LIA-1 standeard, which I can't find > > with google.
C99 defines unsigned overflow semantics, but it doesn't say anything about signed overflow, thus it's undefined -- and you have a hard time finding it out.
However, I have no clue *why* it's undefined and not implementation defined. Does someone know?
> I don't worry about non-2's-complement machines (they don't exist, and > likely won't exist in the future either).
I think DSPs can do saturated arithmetics (clamp to min/max values instead of wrap around). Not that it matters for Linux...
> So I worry about compilers rewriting my code.
gcc has -fwrapv and -ftrapv to change signed integer overflow behaviour.
One baffling example where gcc rewrites code is when conditionals depend on signed integer overflow:
$ cat xx.c #include <assert.h>
int foo(int a) { assert(a + 100 > a); return a; }
int bar(int a) { if (a + 100 > a) a += 100; return a; } $ gcc -Wall -Wextra -fomit-frame-pointer -c xx.c $ objdump -dr xx.o
xx.o: file format elf32-i386
Disassembly of section .text:
00000000 <foo>: 0: 8b 44 24 04 mov 0x4(%esp),%eax 4: c3 ret
00000005 <bar>: 5: 83 44 24 04 64 addl $0x64,0x4(%esp) a: 8b 44 24 04 mov 0x4(%esp),%eax e: c3 ret
The assert and the condition were just dropped by gcc -- without any warning.
gcc-4.2 will add -fstrict-overflow and -Wstrict-overflow. http://gcc.gnu.org/gcc-4.2/changes.html
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