[Bug middle-end/38204] PRE for post dominating expressions

["steven at gcc dot gnu dot org" <gcc-bugzilla@gcc.gnu.org>]

------- Comment #4 from steven at gcc dot gnu dot org  2008-12-01 22:12 -------
davidxl, my latest patch to bug 23286 (tree-hoist_v3.diff) makes PRE in
tree-ssa-pre.c code-size aware.  I have tested this with CSiBE on
mips-unknown-elf.

Without the patch, code size at -Os with PRE enabled is ~17% larger than -Os
without PRE.

With the patch, the code size increase with -Os and PRE enabled is 0.2%.  

With the patch and with tree hoisting enabled at -Os, code size is reduced by
just a little less than 1%.

With the patch and with tree hoisting enabled at -Os but RTL code hoisting
disabled (i.e. the pass in gcse.c), code size is reduced by just a little less
than 0.6%.  (The RTL pass will be necessary mostly to hoist copies from
out-of-ssa, and frame/stack pointer adjustments.)

davidxl, if you have some time or someone else with time to play with this
further, that'd be much appreciated.  I've done my part, but having done the
trick, the fun is gone and I have no intention to persue code-size aware PRE
further ;-)


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38204