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Date: Mon, 23 Oct 2006 16:28:51 +0200
From: Avi Kivity <avi@qumranet.com>
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Subject: Re: [Qemu-devel] qemu vs gcc4
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	<200610231320.08914.paul@codesourcery.com>
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To: Paul Brook <paul@codesourcery.com>
Cc: qemu-devel@nongnu.org

Paul Brook wrote:
>>> That's exactly what my gcc4 hacks do.
>>>
>>> It gets complicated because a x86 uses variable length insn encodings so
>>> you don't know where insn boundaries are, and a jmp instruction is larger
>>> than a ret instruction so it's not always possible to do a straight
>>> replacement.
>>>       
>> how about
>>
>> void some_generated_instruction(u32 a1, u32 s2)
>> {
>>        // code
>>        asm volatile ( "" );
>> }
>>
>>
>> that will force the code to fall through to the null asm code, avoiding
>> premature returns.
>>
>> if the code uses 'return' explicitly, turn it to a goto just before the
>> 'asm volatile'.
>>     
>
> We already do that. It doesn't stop gcc putting the return in the middle of 
> the function.
>
> Paul
>   
void f1();
void f2();

void f(int *z, int x, int y)
{
    if (x) {
        *z = x;
        f1();
    } else {
        *z = y;
        f2();
    }
    asm volatile ("");
}

works, with gcc -O2 -fno-reorder-blocks. removing either the asm or the 
-f flag doesn't.  No idea if it's consistent across architectures.

(the function calls are there to prevent cmov optimizations)




-- 
error compiling committee.c: too many arguments to function