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Re: RFA: recognize new instructions in rs6000 prologues
On 26 Mar 2004 11:56:14 -0500
Jim Blandy <jimb@redhat.com> wrote:
> Kevin Buettner <kevinb@redhat.com> writes:
> > On 24 Mar 2004 10:10:04 -0500
> > Jim Blandy <jimb@redhat.com> wrote:
> >
> > >
> > > These prologues are generated by a not-yet-released compiler, but the
> > > test suite does catch the problem.
> > >
> > > 2004-02-25 Jim Blandy <jimb@redhat.com>
> > >
> > > * rs6000-tdep.c (skip_prologue): Recognize moves from argument
> > > registers to temp register r0 and byte stores as prologue
> > > instructions.
> > >
> > > *** gdb/rs6000-tdep.c.~2~ 2004-02-25 15:14:13.000000000 -0500
> > > --- gdb/rs6000-tdep.c 2004-02-25 15:15:43.000000000 -0500
> > > ***************
> > > *** 772,777 ****
> > > --- 772,785 ----
> > >
> > > /* store parameters in stack */
> > > }
> > > + /* Move parameters from argument registers to temporary register. */
> > > + else if ((op & 0xfc0007fe) == 0x7c000378 && /* mr(.) Rx,Ry */
> > > + (((op >> 21) & 31) >= 3) && /* R3 >= Ry >= R10 */
> > > + (((op >> 21) & 31) <= 10) &&
> > > + (((op >> 16) & 31) == 0)) /* Rx: scratch register r0 */
> > > + {
> > > + continue;
> > > + }
> >
> > Is this case really needed? I would have thought that the catchall
> > case at the end would handle this situation. I'm concerned that
> > adding this case may cause us to overshoot the prologue in some
> > circumstances. (Of course, there's a danger of doing that anyway...)
>
> Here's the code for the function in question:
>
> .align 2
> .globl arg_passing_test2
> .type arg_passing_test2, @function
> arg_passing_test2:
> .LFB107:
> .loc 1 62 0
> stwu 1,-64(1)
> .LCFI11:
> stw 31,60(1)
> .LCFI12:
> mr 31,1
> .LCFI13:
> mr 0,3
> evstdd 4,16(31)
> stw 5,24(31)
> stw 7,32(31)
> stw 8,36(31)
> stw 9,40(31)
> stb 0,8(31)
> lwz 11,0(1)
> lwz 31,-4(11)
> mr 1,11
> blr
> .LFE107:
> .size arg_passing_test2, .-arg_passing_test2
>
> The first 'lwz' is the first non-prologue instruction; the stores
> above it are argument spills.
>
> You're referring to the catchall case that says that we scan past
> unfamiliar, non-branch instructions unless the frame is already set
> up, right? This function leaves its return address in LR, so there's
> no need to wait for a return address save instruction. And the frame
> pointer is set up by the time we see the 'mr 0,3' instruction. So
> that catch-all case doesn't apply here.
>
> According to the ABI, upon function entry, r0 is caller-saves, and not
> used for passing arguments, so its dead; a move into r0 can't destroy
> information. Moving an argument register into it could at worst be an
> initialization of a variable that would be done sooner than expected.
>
> But missing those spills is a serious problem. Most users don't
> consider their frame 'set up' if GDB displays the arguments as
> garbage. :)
Okay, I'm convinced. This part is approved too.
Kevin