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Re: RFA: recognize new instructions in rs6000 prologues
- From: Jim Blandy <jimb at redhat dot com>
- To: Kevin Buettner <kevinb at redhat dot com>
- Cc: gdb-patches at sources dot redhat dot com
- Date: 26 Mar 2004 11:56:14 -0500
- Subject: Re: RFA: recognize new instructions in rs6000 prologues
- References: <vt2hdwexovn.fsf@zenia.home> <20040325221648.77308418@saguaro>
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. :)
(Of course, the best solution would be for GCC to emit location lists.
Then GDB wouldn't need to care whether arguments were spilled or not:
wherever the program was stopped, it would be able to find them.)
> > else if ((op & 0xfc1f0003) == 0xf8010000 || /* std rx,NUM(r1) */
> > (op & 0xfc1f0000) == 0xd8010000 || /* stfd Rx,NUM(r1) */
> > (op & 0xfc1f0000) == 0xfc010000) /* frsp, fp?,NUM(r1) */
> > ***************
> > *** 781,790 ****
> > /* store parameters in stack via frame pointer */
> > }
> > else if (framep &&
> > ! ((op & 0xfc1f0000) == 0x901f0000 || /* st rx,NUM(r1) */
> > ! (op & 0xfc1f0000) == 0xd81f0000 || /* stfd Rx,NUM(r1) */
> > ! (op & 0xfc1f0000) == 0xfc1f0000))
> > ! { /* frsp, fp?,NUM(r1) */
> > continue;
> >
> > /* Set up frame pointer */
> > --- 789,799 ----
> > /* store parameters in stack via frame pointer */
> > }
> > else if (framep &&
> > ! ((op & 0xfc1f0000) == 0x901f0000 || /* st rx,NUM(r31) */
> > ! (op & 0xfc1f0000) == 0x981f0000 || /* stb Rx,NUM(r31) */
> > ! (op & 0xfc1f0000) == 0xd81f0000 || /* stfd Rx,NUM(r31) */
> > ! (op & 0xfc1f0000) == 0xfc1f0000)) /* frsp, fp?,NUM(r31) */
> > ! {
> > continue;
> >
> > /* Set up frame pointer */
>
> This part is okay.
Great, thanks.