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[SystemZ] Split out comparison elimination into a separate pass

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Perhaps predictably, doing comparison elimination on the fly during
SystemZLongBranch turned out to be a bad idea.  The next patches make
use of LOAD AND TEST and BRANCH ON COUNT, both of which require
changes to earlier instructions.

No functionality change intended.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@187718 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Richard Sandiford
2013-08-05 10:58:53 +00:00
parent 13e6e9171f
commit 66fbb47818
5 changed files with 346 additions and 270 deletions
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22
lib/Target/SystemZ/SystemZTargetMachine.cpp
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@ -65,6 +65,28 @@ bool SystemZPassConfig::addPreSched2() {
}
bool SystemZPassConfig::addPreEmitPass() {
// We eliminate comparisons here rather than earlier because some
// transformations can change the set of available CC values and we
// generally want those transformations to have priority. This is
// especially true in the commonest case where the result of the comparison
// is used by a single in-range branch instruction, since we will then
// be able to fuse the compare and the branch instead.
//
// For example, two-address NILF can sometimes be converted into
// three-address RISBLG. NILF produces a CC value that indicates whether
// the low word is zero, but RISBLG does not modify CC at all. On the
// other hand, 64-bit ANDs like NILL can sometimes be converted to RISBG.
// The CC value produced by NILL isn't useful for our purposes, but the
// value produced by RISBG can be used for any comparison with zero
// (not just equality). So there are some transformations that lose
// CC values (while still being worthwhile) and others that happen to make
// the CC result more useful than it was originally.
//
// Doing it so late makes it more likely that a register will be reused
// between the comparison and the branch, but it isn't clear whether
// preventing that would be a win or not.
if (getOptLevel() != CodeGenOpt::None)
addPass(createSystemZElimComparePass(getSystemZTargetMachine()));
addPass(createSystemZLongBranchPass(getSystemZTargetMachine()));
return true;
}