[llvm-commits] [llvm] r106291 - /llvm/trunk/lib/CodeGen/IfConversion.cpp

[Bob Wilson]

Author: bwilson
Date: Fri Jun 18 12:07:23 2010
New Revision: 106291

URL: http://llvm.org/viewvc/llvm-project?rev=106291&view=rev
Log:
Fix PR7372: Conditional branches (at least on ARM) are treated as predicated,
so when IfConverter::CopyAndPredicateBlock checks to see if it should ignore
an instruction because it is a branch, it should not check if the branch is
predicated.

This case (when IgnoreBr is true) is only relevant from IfConvertTriangle,
where new branches are inserted after the block has been copied and predicated.
If the original branch is not removed, we end up with multiple conditional
branches (possibly conflicting) at the end of the block.  Aside from any
immediate errors resulting from that, this confuses the AnalyzeBranch functions
so that the branches are not analyzable.  That in turn causes the IfConverter to
think that the "Simple" pattern can be applied, and things go downhill fast
because the "Simple" pattern does _not_ apply if the block can fall through.

This is pretty fragile.  If there are other degenerate cases where AnalyzeBranch
fails, but where the block may still fall through, the IfConverter should not
perform its "Simple" if-conversion.  But, I don't know how to do that with the
current AnalyzeBranch interface, so for now, the best thing seems to be to
avoid creating branches that AnalyzeBranch cannot handle.

Evan, please review!

Modified:
    llvm/trunk/lib/CodeGen/IfConversion.cpp

Modified: llvm/trunk/lib/CodeGen/IfConversion.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/CodeGen/IfConversion.cpp?rev=106291&r1=106290&r2=106291&view=diff
==============================================================================
--- llvm/trunk/lib/CodeGen/IfConversion.cpp (original)
+++ llvm/trunk/lib/CodeGen/IfConversion.cpp Fri Jun 18 12:07:23 2010
@@ -1298,16 +1298,15 @@
   for (MachineBasicBlock::iterator I = FromBBI.BB->begin(),
          E = FromBBI.BB->end(); I != E; ++I) {
     const TargetInstrDesc &TID = I->getDesc();
-    bool isPredicated = TII->isPredicated(I);
     // Do not copy the end of the block branches.
-    if (IgnoreBr && !isPredicated && TID.isBranch())
+    if (IgnoreBr && TID.isBranch())
       break;
 
     MachineInstr *MI = MF.CloneMachineInstr(I);
     ToBBI.BB->insert(ToBBI.BB->end(), MI);
     ToBBI.NonPredSize++;
 
-    if (!isPredicated && !MI->isDebugValue()) {
+    if (!TII->isPredicated(I) && !MI->isDebugValue()) {
       if (!TII->PredicateInstruction(MI, Cond)) {
 #ifndef NDEBUG
         dbgs() << "Unable to predicate " << *I << "!\n";