Re: continuing egcs-1.1 problems

[Elgin Lee <ehl at funghi dot com>]

>>>>> In <Pine.LNX.3.96.981031152626.20552A-100000@keeshond.aerodyne.com> 
>>>>>	Fred Bacon <bacon@aerodyne.com> wrote:

> The trampoline patch fixes a couple of problems with verifying the size of
> the stack and keeping the stack aligned.  My best guess is that this is
> the critical patch, but I haven't confirmed it.  Someone with a little
> patience needs to rebuild egcs with these patches one at a time to see
> which one really fixes the problem.  Frankly, I've built egcs five times
> in the last 36 hours, and I'm sick of the sight of it. :-)

I think that this is the critical path.  In particular,

--- egcs-1.0/gcc/config/rs6000/tramp.asm~	Mon Aug 11 17:57:23 1997
+++ egcs-1.0/gcc/config/rs6000/tramp.asm	Fri Feb 27 06:11:10 1998
@@ -85,7 +85,7 @@
         lwz	r7,.Ltramp(r11)		/* trampoline addres -4 */
 
 	li	r8,__trampoline_size	/* verify that the trampoline is big enough */
-	cmpw	cr1,r8,r4
+	cmpw	cr1,r4,r8
 	srwi	r4,r4,2			/* # words to move */
 	addi	r9,r3,-4		/* adjust pointer for lwzu */
 	mtctr	r4

is definitely needed.  When the test program enters this path,
r8 (__trampoline_size) is 40 but r4 is 48--hence the branch

	blt	cr1,.Labort

jumps to the abort code.

I'm hazy on the workings of the code, but on the surface this looks
like a comparison-reversal bug that is fixed by the patch.  (But why
does this work on the RS6000/AIX?  Perhaps because r4 and r8 are equal
there on the RS6000/AIX?)

The core dump stems from the abort() call that's made in __trampoline_setup.
See gcc/config/rs6000/tramp.asm.  If the trampoline is not considered
large enough, that routine will call abort()--and that's precisely what's
happening.  You can see for yourself by putting a breakpoint at
__trampoline_setup and then re-running your test program.

Note that the default (nss) implementation in glibc uses a nested
function, which requires a trampoline.  See the definition of
nss_lookup_function in nss/nsswitch.c in the glibc source--it
dynamically uses the nested function do_open() to handle the opening of
the appropriate shared library for the host lookup.

The libresolv.so implementation, on the other hand, does not use a
nested function in this case.  That's the reason that the program runs
fine when linked explicitly with -lresolv.

Without this patch, I suspect that some other trampolines (and hence,
other nested functions in glibc and elsewhere) would likewize fail.

On the surface, it looks like you should indeed include the particular
patch portion above in the egcs build.  I don't know if the other
portion of the trampoline patch (which causes rs6000_trampoline_size
to return 48 rather than 40) is also necessary.

--Elgin