[MCA] Improve code comment and reuse an helper function in ResourceManager. NFCI

llvm-svn: 350322

1 files changed, 10 insertions, 9 deletions

diff --git a/llvm/lib/MCA/HardwareUnits/ResourceManager.cpp b/llvm/lib/MCA/HardwareUnits/ResourceManager.cpp
index 9fc92cd8185..6c21b771330 100644
--- a/llvm/lib/MCA/HardwareUnits/ResourceManager.cpp
+++ b/llvm/lib/MCA/HardwareUnits/ResourceManager.cpp
@@ -24,11 +24,17 @@ namespace mca {
 #define DEBUG_TYPE "llvm-mca"
 ResourceStrategy::~ResourceStrategy() = default;
 
+// Returns the index of the highest bit set. For resource masks, the position of
+// the highest bit set can be used to construct a resource mask identifier.
+static unsigned getResourceStateIndex(uint64_t Mask) {
+  return std::numeric_limits<uint64_t>::digits - countLeadingZeros(Mask);
+}
+
 static uint64_t selectImpl(uint64_t CandidateMask,
                            uint64_t &NextInSequenceMask) {
-  CandidateMask = 1ULL << (countLeadingZeros(CandidateMask) ^
-                           (std::numeric_limits<uint64_t>::digits - 1));
-  NextInSequenceMask &= (CandidateMask ^ (CandidateMask - 1));
+  // The upper bit set in CandidateMask identifies our next candidate resource.
+  CandidateMask = 1ULL << (getResourceStateIndex(CandidateMask) - 1);
+  NextInSequenceMask &= (CandidateMask | (CandidateMask - 1));
   return CandidateMask;
 }
 
@@ -69,8 +75,7 @@ ResourceState::ResourceState(const MCProcResourceDesc &Desc, unsigned Index,
       BufferSize(Desc.BufferSize), IsAGroup(countPopulation(ResourceMask) > 1) {
   if (IsAGroup) {
     ResourceSizeMask =
-        ResourceMask ^ (1ULL << (countLeadingZeros(ResourceMask) ^
-                                 (std::numeric_limits<uint64_t>::digits - 1)));
+        ResourceMask ^ 1ULL << (getResourceStateIndex(ResourceMask) - 1);
   } else {
     ResourceSizeMask = (1ULL << Desc.NumUnits) - 1;
   }
@@ -103,10 +108,6 @@ void ResourceState::dump() const {
 }
 #endif
 
-static unsigned getResourceStateIndex(uint64_t Mask) {
-  return std::numeric_limits<uint64_t>::digits - countLeadingZeros(Mask);
-}
-
 static std::unique_ptr<ResourceStrategy>
 getStrategyFor(const ResourceState &RS) {
   if (RS.isAResourceGroup() || RS.getNumUnits() > 1)