[DebugInfo][InstrRef][NFC] Cache some PHI resolutions

Install a cache of DBG_INSTR_REF -> ValueIDNum resolutions, for scenarios
where the value has to be reconstructed from several DBG_PHIs. Whenever
this happens, it's because branch folding + tail duplication has messed
with the SSA form of the program, and we have to solve a mini SSA problem
to find the variable value. This is always called twice, so it makes sense
to cache the value.

This gives a ~0.5% geomean compile-time-performance improvement on CTMark.

Differential Revision: https://reviews.llvm.org/D118455

1 files changed, 16 insertions, 0 deletions

diff --git a/llvm/lib/CodeGen/LiveDebugValues/InstrRefBasedImpl.cpp b/llvm/lib/CodeGen/LiveDebugValues/InstrRefBasedImpl.cpp
index 2e69a5e437e7..25d764dee2ed 100644
--- a/llvm/lib/CodeGen/LiveDebugValues/InstrRefBasedImpl.cpp
+++ b/llvm/lib/CodeGen/LiveDebugValues/InstrRefBasedImpl.cpp
@@ -3122,6 +3122,7 @@ bool InstrRefBasedLDV::ExtendRanges(MachineFunction &MF,
   DebugPHINumToValue.clear();
   OverlapFragments.clear();
   SeenFragments.clear();
+  SeenDbgPHIs.clear();
 
   return Changed;
 }
@@ -3387,6 +3388,21 @@ Optional<ValueIDNum> InstrRefBasedLDV::resolveDbgPHIs(MachineFunction &MF,
                                                       ValueIDNum **MLiveIns,
                                                       MachineInstr &Here,
                                                       uint64_t InstrNum) {
+  // This function will be called twice per DBG_INSTR_REF, and might end up
+  // computing lots of SSA information: memoize it.
+  auto SeenDbgPHIIt = SeenDbgPHIs.find(&Here);
+  if (SeenDbgPHIIt != SeenDbgPHIs.end())
+    return SeenDbgPHIIt->second;
+
+  Optional<ValueIDNum> Result =
+      resolveDbgPHIsImpl(MF, MLiveOuts, MLiveIns, Here, InstrNum);
+  SeenDbgPHIs.insert({&Here, Result});
+  return Result;
+}
+
+Optional<ValueIDNum> InstrRefBasedLDV::resolveDbgPHIsImpl(
+    MachineFunction &MF, ValueIDNum **MLiveOuts, ValueIDNum **MLiveIns,
+    MachineInstr &Here, uint64_t InstrNum) {
   // Pick out records of DBG_PHI instructions that have been observed. If there
   // are none, then we cannot compute a value number.
   auto RangePair = std::equal_range(DebugPHINumToValue.begin(),