Re: [PATCH , all targets] Fix order of prologue instructions for varargs functions with -O0 and -m32

[Luis Machado <luisgpm at linux dot vnet dot ibm dot com>]

On Thu, 2009-02-26 at 16:32 +0000, Dave Korn wrote:
> Luis Machado wrote:
> 
> > When built with -O0 and -m32, we get the following asm as output:
> > 
> >         stw 4,28(31)
> >         stw 5,32(31)
> >         stw 6,36(31)
> >         stw 7,40(31)
> >         stw 8,44(31)
> >         stw 9,48(31)
> >         stw 10,52(31)
> >>>>>> Debug info tells us r3 is already saved at this point
> >         bne 1,.L2
> >         stfd 1,56(31)
> >         stfd 2,64(31)
> >         stfd 3,72(31)
> >         stfd 4,80(31)
> >         stfd 5,88(31)
> >         stfd 6,96(31)
> >         stfd 7,104(31)
> >         stfd 8,112(31)
> >>>>>> End of prologue/Beginning of function
> >         stw 3,120(31)
> > 
> > As we can see, r3 isn't saved to the stack until later in the process,
> > but the debugging information tells us r3 should already be on slot 120
> > right before the branch instruction. This prevents GDB from getting the
> > correct value for the last named argument of a variadic function right
> > upon entering it.
> 
>   Wouldn't it be simpler to fix the debug info generation to be correct,
> rather than risk tampering with the code generation on every platform in the
> world?  Also, subject line updated; you're going to need a global reviewer for
> the change to function.c, not an rs6000 backend maintainer.
> 
>   What actual changes result to the generated debug and assembly after your
> patch, how does it look then?
> 
>     cheers,
>       DaveK
> 

The resulting asm code is as follows:

        stw 3,24(31)
        stw 4,36(31)
        stw 5,40(31)
        stw 6,44(31)
        stw 7,48(31)
        stw 8,52(31)
        stw 9,56(31)
        stw 10,60(31)
        bne 1,.L4
        .loc 1 2 0
        stfd 1,64(31)
        stfd 2,72(31)
        stfd 3,80(31)
        stfd 4,88(31)
        stfd 5,96(31)
        stfd 6,104(31)
        stfd 7,112(31)
        stfd 8,120(31)

The debug info tells us r3 is at slot 24, instead of 120 as in the
previous case.

Luis