Change the way we choose a free register: instead of picking the first

free allocatable register, we prefer the a free one with the most uses
of inactive intervals. This causes less spills and performes a bit
better compared to gcc:

Program                 | GCC/LLC (Before)| GCC/LLC (After)
164.gzip/164.gzip       | 0.59            | 0.60
175.vpr/175.vpr         | 0.57            | 0.58
176.gcc/176.gcc         | 0.59            | 0.61
181.mcf/181.mcf         | 0.94            | 0.95
186.crafty/186.crafty   | 0.62            | 0.62
197.parser/197.parser   | 0.89            | 0.88
252.eon/252.eon         | 0.61            | 0.66
253.perlbmk/253.perlbmk | 0.79            | 0.84
254.gap/254.gap         | 0.81            | 0.81
255.vortex/255.vortex   | 0.92            | 0.93
256.bzip2/256.bzip2     | 0.69            | 0.69
300.twolf/300.twolf     | 0.91            | 0.90


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@16147 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 14 insertions, 3 deletions

diff --git a/lib/CodeGen/RegAllocLinearScan.cpp b/lib/CodeGen/RegAllocLinearScan.cpp
index 0204494fd6..00e4e8979e 100644
--- a/lib/CodeGen/RegAllocLinearScan.cpp
+++ b/lib/CodeGen/RegAllocLinearScan.cpp
@@ -522,15 +522,26 @@ void RA::assignRegOrStackSlotAtInterval(LiveInterval* cur)
 
 unsigned RA::getFreePhysReg(LiveInterval* cur)
 {
+  std::vector<unsigned> inactiveCounts(mri_->getNumRegs(), 0);
+  for (IntervalPtrs::iterator i = inactive_.begin(), e = inactive_.end();
+       i != e; ++i) {
+    unsigned reg = (*i)->reg;
+    if (MRegisterInfo::isVirtualRegister(reg))
+      reg = vrm_->getPhys(reg);
+    ++inactiveCounts[reg];
+  }
+
   const TargetRegisterClass* rc = mf_->getSSARegMap()->getRegClass(cur->reg);
 
+  unsigned freeReg = 0;
   for (TargetRegisterClass::iterator i = rc->allocation_order_begin(*mf_);
        i != rc->allocation_order_end(*mf_); ++i) {
     unsigned reg = *i;
-    if (prt_->isRegAvail(reg))
-      return reg;
+    if (prt_->isRegAvail(reg) &&
+        (!freeReg || inactiveCounts[freeReg] < inactiveCounts[reg]))
+        freeReg = reg;
   }
-  return 0;
+  return freeReg;
 }
 
 FunctionPass* llvm::createLinearScanRegisterAllocator() {