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Re: Patches to fix optimizer bug & C++ exceptions for GCC VAX backend

From: Jeff Law <law at redhat dot com>
To: Jake Hamby <jehamby420 at me dot com>, Bernd Schmidt <bschmidt at redhat dot com>
Cc: Christos Zoulas <christos at astron dot com>, port-vax at netbsd dot org, gcc-patches at gcc dot gnu dot org, matt at 3am-software dot com
Date: Tue, 26 Apr 2016 14:41:50 -0600
Subject: Re: Patches to fix optimizer bug & C++ exceptions for GCC VAX backend
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References: <F48D0C6B-A6DB-410B-BC97-C30D4E8B4612 at me dot com> <ncu3pc$gkd$1 at ger dot gmane dot org> <DB0276A6-0A26-44F7-8FDC-D086EA40C709 at me dot com> <26D90386-F298-4EDD-A7D7-278F87DF3949 at me dot com> <64F034DF-BDB0-44F3-B4DB-937DEE2795E6 at me dot com> <009114AB-9D39-4A5B-9935-70C012855DB0 at me dot com> <244703AA-2698-41B6-824F-999E1BB61968 at me dot com> <46F08A58-7790-4D2E-8146-9E6B5A3930B0 at me dot com> <56FE5D89 dot 30907 at redhat dot com> <F21E9B35-9A1D-46AF-A0C9-2330D8331069 at me dot com>

On 04/01/2016 01:06 PM, Jake Hamby wrote:


My theory is that it has to do with liveness detection and a write
into the stack frame being incorrectly seen as updating a local
variable, but that could be completely wrong. I was hoping that by
cc'ing gcc-patches that somebody more familiar with why some phase of
the optimizer might decide to delete this particular insn that copies
data into the stack (to overwrite the return address, e.g. to move to
EH_RETURN_HANDLER_RTX) immediately before returning.

Dead store elimination is the most likely candidate. It "knows" thatstores into the local frame are dead when the function returns and usesthat information to eliminate such stores.

You may just need to set the volatile flag on the MEM when you generateit in your backend. For example seeconfig/pa/pa.c::pa_eh_return_handler_rtx.


Jeff

References:
- Re: Patches to fix optimizer bug & C++ exceptions for GCC VAX backend
  - From: Bernd Schmidt
- Re: Patches to fix optimizer bug & C++ exceptions for GCC VAX backend
  - From: Jake Hamby

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