Messages in this thread | | | From | David Howells <> | Subject | Re: Re[2]: [PATCH] fork_init: fix division by zero | Date | Wed, 10 Dec 2008 13:06:44 +0000 |
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Yuri Tikhonov <yur@emcraft.com> wrote:
> Here we believe in preprocessor: since all PAGE_SIZE, 8, and > THREAD_SIZE are the constants we expect it will calculate this.
The preprocessor shouldn't be calculating this. I believe it will _only_ calculate expressions for #if. In the situation you're referring to, it should perform a substitution and nothing more. The preprocessor doesn't necessarily know how to handle the types involved.
In any case, there's an easy way to find out: you can ask the compiler to give you the result of running the source through the preprocessor only. For instance, if you run this:
#define PAGE_SIZE 4096 #define THREAD_SIZE 8192 unsigned long mempages; unsigned long jump(void) { unsigned long max_threads; max_threads = mempages * PAGE_SIZE / (8 * THREAD_SIZE); return max_threads; }
through "gcc -E", you get:
# 1 "calc.c" # 1 "<built-in>" # 1 "<command line>" # 1 "calc.c" unsigned long mempages; unsigned long jump(void) { unsigned long max_threads; max_threads = mempages * 4096 / (8 * 8192); return max_threads; }
> In any case, adding braces as follows probably would be better: > > + max_threads = mempages * (PAGE_SIZE / (8 * THREAD_SIZE));
I think you mean brackets, not braces '{}'.
> Right ?
Definitely not.
I added this function to the above:
unsigned long alt(void) { unsigned long max_threads; max_threads = mempages * (PAGE_SIZE / (8 * THREAD_SIZE)); return max_threads; }
and ran it through "gcc -S -O2" for x86_64:
jump: movq mempages(%rip), %rax salq $12, %rax shrq $16, %rax ret alt: xorl %eax, %eax ret
Note the difference? In jump(), x86_64 first multiplies mempages by 4096, and _then_ divides by 8*8192.
In alt(), it just returns 0 because the compiler realised that you're multiplying by 0.
If you're going to bracket the expression, it must be:
max_threads = (mempages * PAGE_SIZE) / (8 * THREAD_SIZE);
which should be superfluous.
> E.g. here is the result from this line as produced by cross-gcc > 4.2.2: > > lis r9,0 > rlwinm r29,r29,2,16,29 > stw r29,0(r9) > > As you see - only rotate-left, i.e. multiplication to the constant.
Ummm... On powerpc, I believe rotate-left would be a division as it does the bit-numbering and the bit direction the opposite way to more familiar CPUs such as x86.
David
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