Hi,
nothing changes if everything is unsigned and we are guaranteed to not
raise UB on overflow:
unsigned foo(unsigned char *t, unsigned char *v, unsigned w)
{
unsigned i;
for (i = 1; i != w; ++i)
{
unsigned x = i << 2;
v[x + 4] = t[x + 4];
}
return 0;
}
yields:
.L5:
leal 0(,%eax,4), %edx
addl $1, %eax
movzbl 4(%edi,%edx), %ecx
cmpl %ebx, %eax
movb %cl, 4(%esi,%edx)
jne .L5
What is SCEV infrastructure (guessing scalar evolutions?) and what
files/passes to look in?
---
With best regards, Konstantin
On Fri, Dec 6, 2013 at 2:10 PM, Richard Biener
<richard.guent...@gmail.com> wrote:
> On Fri, Dec 6, 2013 at 9:30 AM, Konstantin Vladimirov
> <konstantin.vladimi...@gmail.com> wrote:
>> Hi,
>>
>> Consider code:
>>
>> int foo(char *t, char *v, int w)
>> {
>> int i;
>>
>> for (i = 1; i != w; ++i)
>> {
>> int x = i << 2;
>> v[x + 4] = t[x + 4];
>> }
>>
>> return 0;
>> }
>>
>> Compile it to x86 (I used both gcc 4.7.2 and gcc 4.8.1) with options:
>>
>> gcc -O2 -m32 -S test.c
>>
>> You will see loop, formed like:
>>
>> .L5:
>> leal 0(,%eax,4), %edx
>> addl $1, %eax
>> movzbl 4(%edi,%edx), %ecx
>> cmpl %ebx, %eax
>> movb %cl, 4(%esi,%edx)
>> jne .L5
>>
>> But it can be easily simplified to something like this:
>>
>> .L5:
>> addl $1, %eax
>> movzbl (%esi,%eax,4), %edx
>> cmpl %ecx, %eax
>> movb %dl, (%ebx,%eax,4)
>> jne .L5
>>
>> (i.e. left shift may be moved to address).
>>
>> First question to gcc-help maillist. May be there are some options,
>> that I've missed, and there IS a way to explain gcc my intention to do
>> this?
>>
>> And second question to gcc developers mail list. I am working on
>> private backend and want to add this optimization to my backend. What
>> do you advise me to do -- custom gimple pass, or rtl pass, or modify
>> some existent pass, etc?
>
> This looks like a deficiency in induction variable optimization. Note
> that i << 2 may overflow and this overflow does not invoke undefined
> behavior but is in the implementation defined behavior category.
>
> The issue in this case is likely that the SCEV infrastructure does not handle
> left-shifts.
>
> Richard.
>
>> ---
>> With best regards, Konstantin
