Add option for disabling extra coalescing in VC backend

Add option that disables coalescing of instruction destination live
range with some of its sources (this prevents creation of big live
ranges and may decrease register pressure)

Author: skachkov-intel

IGC/VectorCompiler/igcdeps/src/TranslationInterface.cpp

@@ -209,6 +209,8 @@ template <typename T> T deriveDefaultableFlagValue(int Flag) {
 static void adjustTransformationsAndOptimizations(vc::CompileOptions &Opts) {
   if (IGC_IS_FLAG_ENABLED(VCLocalizeAccUsage))
     Opts.ForceLiveRangesLocalizationForAccUsage = true;
+  if (IGC_IS_FLAG_ENABLED(VCDisableExtraCoalescing))
+    Opts.ForceDisableExtraCoalescing = true;
   if (IGC_IS_FLAG_ENABLED(VCDisableNonOverlappingRegionOpt))
     Opts.ForceDisableNonOverlappingRegionOpt = true;
   if (IGC_IS_FLAG_ENABLED(VCSaveStackCallLinkage))

IGC/VectorCompiler/include/vc/Driver/Driver.h

@@ -87,6 +87,7 @@ struct CompileOptions {
 
   llvm::Optional<unsigned> StackMemSize;
   bool ForceLiveRangesLocalizationForAccUsage = false;
+  bool ForceDisableExtraCoalescing = false;
   bool ForceDisableNonOverlappingRegionOpt = false;
   bool IsLargeGRFMode = false;
   DisableLRCoalescingControl DisableLRCoalescingMode =

IGC/VectorCompiler/include/vc/Support/BackendConfig.h

@@ -89,6 +89,9 @@ struct GenXBackendOptions {
   // Disable LR coalescing
   bool DisableLiveRangesCoalescing = false;
 
+  // Disable extra coalescing
+  bool DisableExtraCoalescing = false;
+
   // Disable non-overlapping region transformation (the case with undef
   // value in two-address operand)
   bool DisableNonOverlappingRegionOpt;
@@ -240,6 +243,10 @@ class GenXBackendConfig : public ImmutablePass {
     return Options.DisableLiveRangesCoalescing;
   }
 
+  bool disableExtraCoalescing() const {
+    return Options.DisableExtraCoalescing;
+  }
+
   bool disableNonOverlappingRegionOpt() const {
     return Options.DisableNonOverlappingRegionOpt;
   }

IGC/VectorCompiler/lib/Driver/Driver.cpp

@@ -247,6 +247,8 @@ static GenXBackendOptions createBackendOptions(const vc::CompileOptions &Opts) {
   BackendOpts.ForceArrayPromotion = (Opts.Binary == vc::BinaryKind::CM);
   if (Opts.ForceLiveRangesLocalizationForAccUsage)
     BackendOpts.LocalizeLRsForAccUsage = true;
+  if (Opts.ForceDisableExtraCoalescing)
+    BackendOpts.DisableExtraCoalescing = true;
   if (Opts.ForceDisableNonOverlappingRegionOpt)
     BackendOpts.DisableNonOverlappingRegionOpt = true;
   BackendOpts.FCtrl = Opts.FCtrl;

IGC/VectorCompiler/lib/GenXCodeGen/GenXVisaRegAlloc.cpp

@@ -145,7 +145,8 @@ bool GenXVisaRegAlloc::runOnFunctionGroup(FunctionGroup &FGArg)
   CurrentRegId[vc::RegCategory::Predicate] = VISA_NUM_RESERVED_PREDICATES;
   CurrentRegId[vc::RegCategory::Surface] = VISA_NUM_RESERVED_SURFACES;
   // Do some extra coalescing.
-  extraCoalescing();
+  if (!BackendConfig->disableExtraCoalescing())
+    extraCoalescing();
   // Get the live ranges in a reproducible order.
   std::vector<LiveRange *> LRs;
   getLiveRanges(LRs);

IGC/VectorCompiler/lib/Support/BackendConfig.cpp

@@ -83,6 +83,10 @@ static cl::opt<bool>
     DisableLRCoalescingOpt("vc-disable-coalescing", cl::init(false), cl::Hidden,
                            cl::desc("disable coalescing of live ranges"));
 
+static cl::opt<bool>
+    DisableExtraCoalescingOpt("vc-disable-extra-coalescing", cl::init(false), cl::Hidden,
+                              cl::desc("disable extrac coalescing"));
+
 static cl::opt<bool> DisableNonOverlappingRegionOptOpt(
     "vc-disable-non-overlapping-region-opt", cl::init(false), cl::Hidden,
     cl::desc("Disable non-overlapping region optimization"));
@@ -146,6 +150,7 @@ GenXBackendOptions::GenXBackendOptions()
       UseNewStackBuilder(UseNewStackBuilderOpt),
       LocalizeLRsForAccUsage(LocalizeLRsForAccUsageOpt),
       DisableLiveRangesCoalescing(DisableLRCoalescingOpt),
+      DisableExtraCoalescing(DisableLRCoalescingOpt),
       DisableNonOverlappingRegionOpt(DisableNonOverlappingRegionOptOpt),
       FCtrl(FunctionControlOpt), IsLargeGRFMode(LargeGRFModeOpt),
       UseBindlessBuffers(UseBindlessBuffersOpt),

IGC/common/igc_flags.h

@@ -579,6 +579,7 @@ DECLARE_IGC_GROUP("VectorCompiler Options")
     DECLARE_IGC_REGKEY(debugString, VCApiOptions, 0, "Extra API options for VC", true)
     DECLARE_IGC_REGKEY(debugString, VCInternalOptions, 0, "Extra Internal options to pass to VC", true)
     DECLARE_IGC_REGKEY(bool, VCLocalizeAccUsage, false, "Localization of possible accumulator usages for vISA RA", true)
+    DECLARE_IGC_REGKEY(bool, VCDisableExtraCoalescing, false, "Disable extra coalescing", true)
     DECLARE_IGC_REGKEY(bool, VCDisableNonOverlappingRegionOpt, false, "Disable non-overlapping region optimization", true)
     DECLARE_IGC_REGKEY(bool, VCEnableExtraDebugLogging, false, "Turns on extra debug output to trace IGC/VC-specific execution", true)
     DECLARE_IGC_REGKEY(DWORD, VCNoOptFinalizerControl, 0, "Controls if finalizer is invoked with -debug flag", true)