[Bug rtl-optimization/46793] New: -fschedule-insns causes ICE in compiling zlib/trees.c

[Bug rtl-optimization/46793] New: -fschedule-insns causes ICE in compiling zlib/trees.c

[changpeng.fang at amd dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=46793

           Summary: -fschedule-insns causes ICE in compiling zlib/trees.c
           Product: gcc
           Version: 4.6.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: rtl-optimization
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: changpeng.fang@amd.com


-fschedule-insns causes ICE in compiling zlib/trees.c: 
trees.c:701:1: error: unable to find a register to spill in class ‘Q_REGS’
This failure only occurs for -m32 compilation.

To reproduce:
cd trunk/zlib
gcc trees.c -c -o ~/tree.o -O2 -fschedule-insns -m32

[~/gcc-4.6-20101120/zlib]$ gcc trees.c -c -o ~/tree.o -O2 -fschedule-insns -m32
trees.c: In function ‘build_tree’:
trees.c:701:1: error: unable to find a register to spill in class ‘Q_REGS’
trees.c:701:1: error: this is the insn:
(insn 473 429 164 18 (set (subreg:SI (reg:QI 83 [ iftmp.7 ]) 0)
        (if_then_else:SI (ltu (reg:CC 17 flags)
                (const_int 0 [0]))
            (subreg:SI (reg:QI 388) 0)
            (subreg:SI (reg:QI 83 [ iftmp.7 ]) 0))) trees.c:677 854
{*movsicc_noc}
     (expr_list:REG_DEAD (reg:QI 388)
        (expr_list:REG_DEAD (reg:CC 17 flags)
            (nil))))
trees.c:701:1: internal compiler error: in spill_failure, at reload1.c:2105
Please submit a full bug report,
with preprocessed source if appropriate.
See <http://gcc.gnu.org/bugs.html> for instructions.

This should be a regression (I only know 4.3 is working)

[Bug rtl-optimization/46793] -fschedule-insns causes ICE in compiling zlib/trees.c

[steven at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=46793

Steven Bosscher <steven at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
                 CC|                            |steven at gcc dot gnu.org
         Resolution|                            |WONTFIX

--- Comment #1 from Steven Bosscher <steven at gcc dot gnu.org> 2010-12-03 20:36:52 UTC ---
Try with -fsched-pressure?

But -fschedule-insns on 32-bits ix86 is a Really Bad Idea. There are plenty bug
reports for similar failures. There are just not enough registers and too many
special constraints. This is not a fixable issue, beyond keeping
-fschedule-insns disabled.

=> WONTFIX

[Bug rtl-optimization/46793] -fschedule-insns causes ICE in compiling zlib/trees.c

[changpeng.fang at amd dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=46793

--- Comment #2 from Changpeng Fang <changpeng.fang at amd dot com> 2010-12-03 20:52:11 UTC ---
This bug should be fixed.

We are trying to make -fschedule-insns default for x86, and this enablement
causes
bootstrapping failure at this point.

Of course we can work around this by just enabling it for 64 bit.

[Bug rtl-optimization/46793] -fschedule-insns causes ICE in compiling zlib/trees.c

[steven at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=46793

--- Comment #3 from Steven Bosscher <steven at gcc dot gnu.org> 2010-12-03 21:05:46 UTC ---
The bug is in the architecture, not in the scheduling pass. You cannot expect
scheduling before register allocation to always yield code that you can
register-allocate with just 3 general registers available.

Enabling it only for AMD64 is not a work-around, it's the only proper thing to
do if you want to enable the pre-RA scheduling pass by default.

And there is still -fsched-pressure that you could try.

But I doubt you will get a patch approved to enable scheduling for 32-bits
ix86. The failure you are seeing can be triggered quite easily even _without_
scheduling enabled. Just search bugzilla for all the "unable to find spill
register" bugs on i386, this one is far from unique.

[Bug rtl-optimization/46793] -fschedule-insns causes ICE in compiling zlib/trees.c

[changpeng.fang at amd dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=46793

--- Comment #4 from Changpeng Fang <changpeng.fang at amd dot com> 2010-12-03 21:24:06 UTC ---
Thanks, I understand the issue now.
Yes, -fschedule-insns and -fsched-pressure should be a paired options for
x86. -fsched-pressure does not solve the -m32 issue.

[Bug rtl-optimization/46793] -fschedule-insns causes ICE in compiling zlib/trees.c

[joseph at codesourcery dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=46793

--- Comment #5 from joseph at codesourcery dot com <joseph at codesourcery dot com> 2010-12-03 23:04:52 UTC ---
We should disable this option (with a warning) in 
ix86_option_override_internal if we don't think it can work reliably, 
rather than leaving it to ICE.