------- Comment #2 from steven at gcc dot gnu dot org 2007-10-20 10:48 -------
A second issue with code hoisting in gcse.c is that it only looks at basic
blocks immediately dominated by the branch point to hoist to. Quoting
gcse.c:hoist_code():
/* Walk over each basic block looking for potentially hoistable
expressions, nothing gets hoisted from the entry block. */
FOR_EACH_BB (bb)
{
int found = 0;
int insn_inserted_p;
domby = get_dominated_by (CDI_DOMINATORS, bb);
...
An expression can be very busy at some point in the CFG, and be computed in a
block that is not immediately dominated at that point, e.g.
BB1
if (...) goto BB2 else goto BB3
{ succ BB2, BB3 }
{ pred BB1 }
BB2
r1 <- exp1
goto BB6
{ succ BB6 }
{ pred BB1}
BB3
if (...) goto BB4 else goto BB5
{ succ BB4, BB5 }
{ pred BB3 }
BB4
r2 <- exp1
goto BB6
{ succ BB6 }
{ pred BB3 }
BB5
r3 <- exp1
goto BB6
{ succ BB6 }
{ pred BB2, BB4, BB5 }
BB6
// etc.
The expression exp1 is hoistable to BB1, but BB4 and BB5 are not immediately
dominated by BB1, so gcse.c's code hoisting implementation never optimizes
this.
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=33828
