[MCA][RegisterFile] Consistently update the PRF in the presence of multiple writes...

My last change to the RegisterFile (PR51495) has introduced a bug in the logic
that allocates physical registers in the PRF.

In some cases, this bug could have triggered a nasty unsigned wrap in the number
of allocated registers, thus resulting in mca being stuck forever in a loop of
PRF availability checks.

diff --git a/llvm/lib/MCA/HardwareUnits/RegisterFile.cpp b/llvm/lib/MCA/HardwareUnits/RegisterFile.cpp
index 1c0f40d..474bf84 100644 (file)
--- a/llvm/lib/MCA/HardwareUnits/RegisterFile.cpp
+++ b/llvm/lib/MCA/HardwareUnits/RegisterFile.cpp
@@ -294,7 +294,9 @@ void RegisterFile::addRegisterWrite(WriteRef Write,
     const WriteState *OtherWS = OtherWrite.getWriteState();
     if (OtherWS && OtherWrite.getSourceIndex() == Write.getSourceIndex()) {
       if (OtherWS->getLatency() > WS.getLatency()) {
-        // Conservatively keep the slowest write to RegID.
+        // Conservatively keep the slowest write on RegID.
+        if (ShouldAllocatePhysRegs)
+          allocatePhysRegs(RegisterMappings[RegID].second, UsedPhysRegs);
         return;
       }
     }