[Bug rtl-optimization/67778] [6 Regression] ICE on valid code at -O3 on x86_64-linux-gnu in maybe_record_trace_start, at dwarf2cfi.c:2297

[segher at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67778

--- Comment #14 from Segher Boessenkool <segher at gcc dot gnu.org> ---
Author: segher
Date: Fri Dec 11 00:27:14 2015
New Revision: 231552

URL: https://gcc.gnu.org/viewcvs?rev=231552&root=gcc&view=rev
Log:
Fix shrink-wrapping bug (PR67778, PR68634)

After shrink-wrapping has found the "tightest fit" for where to place
the prologue, it tries move it earlier (so that frame saves are run
earlier) -- but without copying any more basic blocks.

Unfortunately a candidate block we select can be inside a loop, and we
will still allow it (because the loop always exits via our previously
chosen block).  We can do that just fine if we make a duplicate of the
block, but we do not want to here.

So we need to detect this situation.  We can place the prologue at a
previous block PRE only if PRE dominates every block reachable from
it, because then we will never need to duplicate that block (it will
always be executed with prologue).


2015-12-11  Segher Boessenkool  <segher@kernel.crashing.org>

        PR rtl-optimization/67778
        PR rtl-optimization/68634
        * shrink-wrap.c (try_shrink_wrapping): Add a comment about why we want
        to put the prologue earlier.  When determining if an earlier block is
        suitable, make sure it dominates every block reachable from it.

Modified:
    trunk/gcc/ChangeLog
    trunk/gcc/shrink-wrap.c