optimization/1945: flow.c:3790 ICE Patch Prologue contains dead register sets

[shane at agendacomputing dot com]

>Number:         1945
>Category:       optimization
>Synopsis:       flow.c:3790 ICE Patch Prologue contains dead register sets
>Confidential:   no
>Severity:       critical
>Priority:       medium
>Responsible:    unassigned
>State:          open
>Class:          ice-on-legal-code
>Submitter-Id:   net
>Arrival-Date:   Mon Feb 12 04:26:00 PST 2001
>Closed-Date:
>Last-Modified:
>Originator:     shane@agendacomputing.com
>Release:        gcc CVS
>Organization:
>Environment:
Compiling cross compiler on i686 for mipsel-linux
>Description:
The RTL generated by
thread_prologue_and_epilogue_insns (insns);
for the prologue contains dead instructions.  In particular
register sets.  The compiler in its present state ICEs, I'm
not sure this is correct behaviour since it depends on the
REG_MAYBE_DEAD flag for proper behaviour which is never set.

I have attached a tar file with two patches, one is an "aggressive" patch,
prepended with "agg", which just changes the ICE to a warning.
The other patch has the same name minus the prepending "agg"
which basically cuts out optimization during threading and
life_analysis of the prologue.

I have not encountered problems with either of these patches
while using them on a mipsel machine which uses the affected
function calls in libstdc++.  (14 warnings are generated from
the agg-* patch, which has been attached as well)

These patches are only short term solutions, I'm sure there
is a good reason for that ICE to be triggered.  Long term
solutions would probably be:
1) Use the REG_MAYBE_DEAD note properly in life_analysis,
and have it trigerred after_reload on the prologue/epilogue/
sibcall prologue/epilogue.  (aka run some note generation
code on the prologue/epilogue to allow pass through for this
potential ICE)
2) Fix RTL generation for prologue so that it doesn't produce
code which would just be deleted later, or sensed as a bug.
>How-To-Repeat:
Pull down a present CVS copy of GCC, and try to compile
a cross compiler for mipsel-linux with c++ enabled.  I do not think
that this is related to c++ code, but rather just a by product
of the more complex code that is in libstdc++-v3.

Without optimization this error is not triggered, and the
resulting code works fine.
>Fix:
Either patch attached will short term fix the problem.
>Release-Note:
>Audit-Trail:
>Unformatted:
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