gcc 3.2.3 x64 negative indexes

[Paul Edwards via Gcc]

Hi.

I am using a slightly modified gcc 3.2.3 for x86_64 and for this code:

int fff(char *x)
{
return (x[-1]);
}


It is generating:

.globl fff
fff:
.LFB2:
movl$4294967295, %eax
movsbl  (%rax,%rcx),%eax
ret


My understanding is that that move of -1 into eax
does NOT sign-extend, and thus results in a very
high index, which causes a crash.

I may have stuffed something up myself.

Anyone know which code governs this behavior?

Thanks. Paul.

Re: gcc 3.2.3 x64 negative indexes

[Jakub Jelinek via Gcc]

On Wed, Feb 07, 2024 at 11:02:51PM +0800, Paul Edwards via Gcc wrote:
> I am using a slightly modified gcc 3.2.3 for x86_64 and for this code:

Don't, gcc 3.2.3 is not supported for more than 20 years already.

> int fff(char *x)
> {
> return (x[-1]);
> }
> 
> 
> It is generating:
> 
> .globl fff
> fff:
> .LFB2:
> movl$4294967295, %eax
> movsbl  (%rax,%rcx),%eax

That said, I can't reproduce it and get
movsbl  -1(%rdi),%eax
ret
from 3.2.3.

Jakub