Re: bootstrap failure, x86: stage1 miscompiles gengenrtl

[Zack Weinberg <zack at wolery dot cumb dot org>]

On Fri, Mar 31, 2000 at 11:04:12AM +1000, Alan Modra wrote:
> On Thu, 30 Mar 2000, Zack Weinberg wrote:
> 
> > stage1/xgcc -Bstage1/ -B/work/inst/i686-pc-linux-gnu/bin/ -c -DIN_GCC
> >     -W -Wall -Wtraditional -O2 -g -W -Wall -Wtraditional -DHAVE_CONFIG_H
> >     -I. -I/work/src/hashtab.gcc/gcc -I/work/src/hashtab.gcc/gcc/config
> >     -I/work/src/hashtab.gcc/gcc/../include
> >     /work/src/hashtab.gcc/gcc/gengenrtl.c
> > stage1/xgcc -Bstage1/ -B/work/inst/i686-pc-linux-gnu/bin/ -DIN_GCC -W
> >      -Wall -Wtraditional -O2 -g -W -Wall -Wtraditional -DHAVE_CONFIG_H 
> >      -o gengenrtl gengenrtl.o obstack.o
> > ./gengenrtl -h >tmp-genrtl.h
> > Segmentation fault

...
> >         movl    8(%ebp), %edi
> >         .stabn 68,0,261,.LM74-genmacro
> > .LM74:
> >         leal    (%edi,%edi,2), %eax
> >         leal    0(,%eax,4), %ebx
> >         movl    defs(%ebx), %eax
> >         pushl   %eax
> >         subl    $12, %esp
> 
> The above two instructions seem to be in the wrong order.  That's likely
> your segv problem.

Yeah, that'd do it.

I know that it worked yesterday, so it's got to be the size-in-bits
changes that broke this, but I don't see anything obvious in the
changelog for that.  The subl comes from the stack alignment code,
presumably.

> > I'm not exactly an x86 assembly expert, but it appears to me that the
> > code generated by 2.95 is multiplying idx by 3 before the 
> > leal 0(,%eax,4) instruction, while the current tree is multiplying it
> > by 4.  sizeof(*defs) is 12; both versions agree on that if explicitly
> > asked.
> 
> Nup.  "leal (%edi,%edi,2), %eax" does eax = edi + edi * 2

Aha.  Much more useful semantics that way, now I think about it.

zw