[Bug rtl-optimization/33828] Issues with code hoisting implementation in gcse.c

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

------- Comment #13 from steven at gcc dot gnu dot org  2008-12-01 12:24 -------
After fixing the issue mentioned in comment#2 and comment #8, gcse.c hoisting
hoists things too far up, e.g.:

{ pred ENTRY }
BB1
if (...) goto BB2 else goto BB3
{ succ BB2, BB3 }

{ pred BB1 }
BB2
...
goto BB4
{ succ BB4 }

{ pred BB2 }
BB3
...
goto BB4
{ succ BB4 }

{ pred BB2, BB3 }
BB4
if (...) goto BB5 else goto BB6
{ succ BB5, BB6 }

{ pred BB4 }
BB5
r1 <- exp1
goto BB7
{ succ BB7 }

{ pred BB4 }
BB6
r2 <- exp1
goto BB7
{ succ BB7 }

{ pred BB5, BB6 }
BB7
...
{ succ EXIT }



is transformed to:



{ pred ENTRY }
BB1
r3 <- exp1
if (...) goto BB2 else goto BB3
{ succ BB2, BB3 }

{ pred BB1 }
BB2
...
goto BB4
{ succ BB4 }

{ pred BB2 }
BB3
...
goto BB4
{ succ BB4 }

{ pred BB2, BB3 }
BB4
if (...) goto BB5 else goto BB6
{ succ BB5, BB6 }

{ pred BB4 }
BB5
r1 <- r3
goto BB7
{ succ BB7 }

{ pred BB4 }
BB6
r2 <- r3
goto BB7
{ succ BB7 }

{ pred BB5, BB6 }
BB7
...
{ succ EXIT }



when it would be better to hoist up only to BB4:


{ pred ENTRY }
BB1
if (...) goto BB2 else goto BB3
{ succ BB2, BB3 }

{ pred BB1 }
BB2
...
goto BB4
{ succ BB4 }

{ pred BB2 }
BB3
...
goto BB4
{ succ BB4 }

{ pred BB2, BB3 }
BB4
r3 <- exp1
if (...) goto BB5 else goto BB6
{ succ BB5, BB6 }

{ pred BB4 }
BB5
r1 <- r3
goto BB7
{ succ BB7 }

{ pred BB4 }
BB6
r2 <- r3
goto BB7
{ succ BB7 }

{ pred BB5, BB6 }
BB7
...
{ succ EXIT }


GCC should not hoist up further than up to the first common dominator.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=33828