[llvm] r297673 - [IPRA] Change algorithm for RegUsageInfoCollector.

[Marcello Maggioni via llvm-commits]

Author: mggm
Date: Mon Mar 13 16:42:53 2017
New Revision: 297673

URL: http://llvm.org/viewvc/llvm-project?rev=297673&view=rev
Log:
[IPRA] Change algorithm for RegUsageInfoCollector.

The previous algorithm for RegUsageInfoCollector had pretty bad
performance on architectures with a lot of registers that alias
a lot one another, because we potentially iterate for every register
over all the aliasing registers. This costs even more if the function
is small and doesn't define a lot of registers.
This patch changes the algorithm to one that while iterating over
all the registers it will iterate over the aliasing registers only
if the register itself is defined.
This should be faster based on the assumption that only a subset
of the whole LLVM registers set is actually defined in the function.

Differential Revision: https://reviews.llvm.org/D30880

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

Modified: llvm/trunk/lib/CodeGen/RegUsageInfoCollector.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/CodeGen/RegUsageInfoCollector.cpp?rev=297673&r1=297672&r2=297673&view=diff
==============================================================================
--- llvm/trunk/lib/CodeGen/RegUsageInfoCollector.cpp (original)
+++ llvm/trunk/lib/CodeGen/RegUsageInfoCollector.cpp Mon Mar 13 16:42:53 2017
@@ -103,9 +103,27 @@ bool RegUsageInfoCollector::runOnMachine
 
   DEBUG(dbgs() << "Clobbered Registers: ");
 
-  for (unsigned PReg = 1, PRegE = TRI->getNumRegs(); PReg < PRegE; ++PReg)
-    if (MRI->isPhysRegModified(PReg, true))
-      RegMask[PReg / 32] &= ~(1u << PReg % 32);
+  const BitVector &UsedPhysRegsMask = MRI->getUsedPhysRegsMask();
+  auto SetRegAsDefined = [&RegMask] (unsigned Reg) {
+    RegMask[Reg / 32] &= ~(1u << Reg % 32);
+  };
+  // Scan all the physical registers. When a register is defined in the current
+  // function set it and all the aliasing registers as defined in the regmask.
+  for (unsigned PReg = 1, PRegE = TRI->getNumRegs(); PReg < PRegE; ++PReg) {
+    // If a register is in the UsedPhysRegsMask set then mark it as defined.
+    // All it's aliases will also be in the set, so we can skip setting
+    // as defined all the aliases here.
+    if (UsedPhysRegsMask.test(PReg)) {
+      SetRegAsDefined(PReg);
+      continue;
+    }
+    // If a register is defined by an instruction mark it as defined together
+    // with all it's aliases.
+    if (!MRI->def_empty(PReg)) {
+      for (MCRegAliasIterator AI(PReg, TRI, true); AI.isValid(); ++AI)
+        SetRegAsDefined(*AI);
+    }
+  }
 
   if (!TargetFrameLowering::isSafeForNoCSROpt(F)) {
     const uint32_t *CallPreservedMask =