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Subject: [Bug middle-end/39326] Segmentation fault with -O1, out of memory with -O2
Date: Sat, 09 Mar 2013 14:58:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39326

--- Comment #43 from Steven Bosscher <steven at gcc dot gnu.org> 2013-03-09 14:57:52 UTC ---
The problem with combine is only "collateral damage" from what dse1 is
doing to this function.  It's loading stored values into registers and
replacing re-loads with those registers, as in:

@@ -747831,6 +780399,7 @@
  474783: r569578:SF=r569577:SF-r177936:SF
       REG_DEAD r569577:SF
       REG_DEAD r177936:SF
+ 605734: r662460:SF=r569578:SF
  474784: [const(`t18571.26403'+0x5fe8)]=r569578:SF
       REG_DEAD r569578:SF
  474786: r177942:SF=[const(`t12303.26335'+0x9008)]
...
  487110: [const(`t24716.26406'+0x2ff0)]=r578824:SF
       REG_DEAD r578824:SF
- 487113: r578827:SF=[const(`t18571.26403'+0x5fe8)]
+ 487113: r578827:SF=r662460:SF
  487114: [const(`t24716.26406'+0x2ff4)]=r578827:SF
       REG_DEAD r578827:SF

The distance between the store and load is large, and that is what is
causing combine to blow up -- walking all the insns between 605734 and
487113 (about 500 of them, which appears to be typical).  This also
increases register pressure, causing the poor IRA/LRA/postreload numbers.

Also not great: dse is loading the same stored value into multiple
registers, e.g.:

@@ -147272,6 +175190,8 @@
  110394: r288307:SF=r288306:SF-r37078:SF
       REG_DEAD r288306:SF
       REG_DEAD r37078:SF
+ 601084: r657810:SF=r288307:SF
+ 601085: r657811:SF=r288307:SF
  110395: [r288302:DI]=r288307:SF
       REG_DEAD r288307:SF

but I'm guessing (and verifying now) that this is cleaned up by fwprop2.