[reg scavenger] Fix the isUsed/isAliasUsed functions so as to not report a false

positive.

In this particular case, R6 was being spilled by the register scavenger when it
was in fact dead.  The isUsed function reported R6 as used because the R6_R7
alias was reserved (due to the fact that we've reserved R7 as the FP).  The
solution is to only check if the original register (i.e., R6) isReserved and
not the aliases.  The aliases are only checked to make sure they're available.

The test case is derived from one of the nightly tester benchmarks and is rather
intractable and difficult to reproduce, so I haven't included it.
rdar://12592448


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@168054 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 5 insertions, 3 deletions

diff --git a/include/llvm/CodeGen/RegisterScavenging.h b/include/llvm/CodeGen/RegisterScavenging.h
index 08d316992e..8752e67a79 100644
--- a/include/llvm/CodeGen/RegisterScavenging.h
+++ b/include/llvm/CodeGen/RegisterScavenging.h
@@ -129,10 +129,12 @@ private:
   /// isReserved - Returns true if a register is reserved. It is never "unused".
   bool isReserved(unsigned Reg) const { return MRI->isReserved(Reg); }
 
-  /// isUsed / isUnused - Test if a register is currently being used.
+  /// isUsed - Test if a register is currently being used.  When called by the
+  /// isAliasUsed function, we only check isReserved if this is the original
+  /// register, not an alias register.
   ///
-  bool isUsed(unsigned Reg) const   {
-    return !RegsAvailable.test(Reg) || isReserved(Reg);
+  bool isUsed(unsigned Reg, bool CheckReserved = true) const   {
+    return !RegsAvailable.test(Reg) || (CheckReserved && isReserved(Reg));
   }
 
   /// isAliasUsed - Is Reg or an alias currently in use?