Re: Bad and/or stupid code for DImode compares with gcc 4.6.1

[Alan Lehotsky <alehotsky at me dot com>]

So, I found the patch to do_jump_by_parts_greater_rtx() by Eric Botcazou that should address the stupid code and the redundant branch.

Should have done a broader search before I wasted email bandwidth...
On Oct 31, 2012, at 1:51 PM, Alan Lehotsky <alehotsky@me.com> wrote:

> I'm looking at code generated for a new port of gcc using 4.6.1 and failing execute/950607-2.c with -O0 only 
> 
> The target chip has only 32 bit instructions, so it's using do_jump_by_parts_<relop>_rtx() to expand the compare.
> I've set up my .md to use the CCmode. 
> 
> I see one case that seems really stupid, and one that's just wrong.  I'm thinking that either I have something really flawed with my port's handing of DImode or that there was a bug in 4.6.1.    The port is only failing about 2100 dejagnu tests (passing 64000+) and a good chunk of the failures are due to the ridiculously small data-memory size of the chip.
> 
> For
> 
> 	long long int x;
> 
>        if ( x < 0 ) return 0 else return 2;
> 
> I see code that compares MSBs and branches on < (less than) as expected.  But then it goes and checks the MSBs for != , and finally it checks the LSBS and emits a conditional branch to  the ELSE, followed by an unconditional branch to the ELSE, so that I end up with code that looks like
> 
> 	mov $r1,x
> 	mov $r2,x+4
>        cmpi $r2,0
>        jlt       .L5
>        cmpi  $r2,0           <=== totally redundant for "x < 0" comparisons
>        jne     .L2
>        cmpi $r1,0
>        jmp     .L4
> 
> .L5 : movi $r1, 0
>         jump .L4
> 
> .L2  : movi $r1, 2
> 
> .L4:
>          ret
> 
> 
> This is a simplification of 950607-2.c, which fails at -O0, but passes at higher optimization levels (go figure...)
>