[PATCH, tree-ssa] Optimize loop invariant phi defs constants

[Christian Bruel]

Hello,

while checking why a loop snippet like

  for (i = 0; i <= 5000; i++)
    if (b)
      a = 2;
    else
      a = x;

was not optimized in -O2 (unless loop unrolling or loop switching), I
found out that the case was already  partially handled by Richard in
PR43934. So this patch just adds a cost to the phi defs constants to
allow the whole test to be hoisted out of the loop.

Richard, does this seem reasonable and OK for 4.10 ?

bootstrapped/regtested for x86

many thanks

Christian





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