[X86] Support branch hint (#97721)

For more details about this feature, please refer to latest Intel 64 and
IA-32 Architectures Optimization Reference Manual Volume 1:
https://www.intel.com/content/www/us/en/content-details/821612/intel-64-and-ia-32-architectures-optimization-reference-manual-volume-1.html

Author: fzou1

clang/lib/Basic/Targets/X86.cpp

@@ -457,6 +457,8 @@ bool X86TargetInfo::handleTargetFeatures(std::vector<std::string> &Features,
       HasCF = true;
     } else if (Feature == "+zu") {
       HasZU = true;
+    } else if (Feature == "+branch-hint") {
+      HasBranchHint = true;
     }
 
     X86SSEEnum Level = llvm::StringSwitch<X86SSEEnum>(Feature)
@@ -1292,6 +1294,7 @@ bool X86TargetInfo::hasFeature(StringRef Feature) const {
       .Case("nf", HasNF)
       .Case("cf", HasCF)
       .Case("zu", HasZU)
+      .Case("branch-hint", HasBranchHint)
       .Default(false);
 }
 

clang/lib/Basic/Targets/X86.h

@@ -174,6 +174,7 @@ class LLVM_LIBRARY_VISIBILITY X86TargetInfo : public TargetInfo {
   bool HasCF = false;
   bool HasZU = false;
   bool HasInlineAsmUseGPR32 = false;
+  bool HasBranchHint = false;
 
 protected:
   llvm::X86::CPUKind CPU = llvm::X86::CK_None;

llvm/lib/Target/X86/X86.td

@@ -749,6 +749,11 @@ def TuningUseGLMDivSqrtCosts
     : SubtargetFeature<"use-glm-div-sqrt-costs", "UseGLMDivSqrtCosts", "true",
         "Use Goldmont specific floating point div/sqrt costs">;
 
+// Starting with Redwood Cove architecture, the branch has branch taken hint
+// (i.e., instruction prefix 3EH).
+def TuningBranchHint: SubtargetFeature<"branch-hint", "HasBranchHint", "true",
+                                        "Target has branch hint feature">;
+
 //===----------------------------------------------------------------------===//
 // X86 CPU Families
 // TODO: Remove these - use general tuning features to determine codegen.
@@ -1124,6 +1129,8 @@ def ProcessorFeatures {
                                                   FeaturePREFETCHI];
   list<SubtargetFeature> GNRFeatures =
     !listconcat(SPRFeatures, GNRAdditionalFeatures);
+  list<SubtargetFeature> GNRAdditionalTuning = [TuningBranchHint];
+  list<SubtargetFeature> GNRTuning = !listconcat(SPRTuning, GNRAdditionalTuning);
 
   // Graniterapids D
   list<SubtargetFeature> GNRDAdditionalFeatures = [FeatureAMXCOMPLEX];
@@ -1815,12 +1822,12 @@ def : ProcModel<"pantherlake", AlderlakePModel,
 def : ProcModel<"clearwaterforest", AlderlakePModel,
                 ProcessorFeatures.CWFFeatures, ProcessorFeatures.ADLTuning>;
 def : ProcModel<"graniterapids", SapphireRapidsModel,
-                ProcessorFeatures.GNRFeatures, ProcessorFeatures.SPRTuning>;
+                ProcessorFeatures.GNRFeatures, ProcessorFeatures.GNRTuning>;
 def : ProcModel<"emeraldrapids", SapphireRapidsModel,
-                ProcessorFeatures.SPRFeatures, ProcessorFeatures.SPRTuning>;
+                ProcessorFeatures.SPRFeatures, ProcessorFeatures.GNRTuning>;
 foreach P = ["graniterapids-d", "graniterapids_d"] in {
 def : ProcModel<P, SapphireRapidsModel,
-                ProcessorFeatures.GNRDFeatures, ProcessorFeatures.SPRTuning>;
+                ProcessorFeatures.GNRDFeatures, ProcessorFeatures.GNRTuning>;
 }
 
 // AMD CPUs.

llvm/lib/Target/X86/X86MCInstLower.cpp

@@ -25,6 +25,7 @@
 #include "llvm/ADT/STLExtras.h"
 #include "llvm/ADT/SmallString.h"
 #include "llvm/ADT/StringExtras.h"
+#include "llvm/CodeGen/MachineBranchProbabilityInfo.h"
 #include "llvm/CodeGen/MachineConstantPool.h"
 #include "llvm/CodeGen/MachineFunction.h"
 #include "llvm/CodeGen/MachineModuleInfoImpls.h"
@@ -54,6 +55,14 @@
 
 using namespace llvm;
 
+static cl::opt<bool> EnableBranchHint("enable-branch-hint",
+                                      cl::desc("Enable branch hint."),
+                                      cl::init(false), cl::Hidden);
+static cl::opt<unsigned> BranchHintProbabilityThreshold(
+    "branch-hint-probability-threshold",
+    cl::desc("The probability threshold of enabling branch hint."),
+    cl::init(50), cl::Hidden);
+
 namespace {
 
 /// X86MCInstLower - This class is used to lower an MachineInstr into an MCInst.
@@ -2444,6 +2453,21 @@ void X86AsmPrinter::emitInstruction(const MachineInstr *MI) {
     if (IndCSPrefix && MI->hasRegisterImplicitUseOperand(X86::R11))
       EmitAndCountInstruction(MCInstBuilder(X86::CS_PREFIX));
     break;
+  case X86::JCC_1:
+    // Two instruction prefixes (2EH for branch not-taken and 3EH for branch
+    // taken) are used as branch hints. Here we add branch taken prefix for
+    // jump instruction with higher probability than threshold.
+    if (getSubtarget().hasBranchHint() && EnableBranchHint) {
+      const MachineBranchProbabilityInfo *MBPI =
+          &getAnalysis<MachineBranchProbabilityInfoWrapperPass>().getMBPI();
+      MachineBasicBlock *DestBB = MI->getOperand(0).getMBB();
+      BranchProbability EdgeProb =
+          MBPI->getEdgeProbability(MI->getParent(), DestBB);
+      BranchProbability Threshold(BranchHintProbabilityThreshold, 100);
+      if (EdgeProb > Threshold)
+        EmitAndCountInstruction(MCInstBuilder(X86::DS_PREFIX));
+    }
+    break;
   }
 
   MCInst TmpInst;

llvm/test/CodeGen/X86/branch-hint.ll

@@ -0,0 +1,75 @@
+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py UTC_ARGS: --version 5
+; RUN: llc < %s -mtriple=x86_64 -mattr=+branch-hint -enable-branch-hint | FileCheck %s
+; RUN: llc < %s -mtriple=x86_64 -mattr=+branch-hint -enable-branch-hint -branch-hint-probability-threshold=50 | FileCheck %s
+; RUN: llc < %s -mtriple=x86_64 -mattr=+branch-hint -enable-branch-hint -branch-hint-probability-threshold=60 -tail-dup-placement=false | FileCheck --check-prefix=TH60 %s
+
+
+; Design: Add DS segment override prefix for condition branch who has high
+; probability to take (which is greater than the probability threshold of
+; enabling branch hint).
+
+define void @p51(i32 %x, ptr %p) {
+; CHECK-LABEL: p51:
+; CHECK:       # %bb.0: # %entry
+; CHECK-NEXT:    testl %edi, %edi
+; CHECK-NEXT:    ds
+; CHECK-NEXT:    je .LBB0_2
+; CHECK-NEXT:  # %bb.1: # %if.then
+; CHECK-NEXT:    movl %edi, (%rsi)
+; CHECK-NEXT:  .LBB0_2: # %if.end
+; CHECK-NEXT:    retq
+;
+; TH60-LABEL: p51:
+; TH60:       # %bb.0: # %entry
+; TH60-NEXT:    testl %edi, %edi
+; TH60-NEXT:    je .LBB0_2
+; TH60-NEXT:  # %bb.1: # %if.then
+; TH60-NEXT:    movl %edi, (%rsi)
+; TH60-NEXT:  .LBB0_2: # %if.end
+; TH60-NEXT:    retq
+entry:
+  %tobool.not = icmp eq i32 %x, 0
+  br i1 %tobool.not, label %if.end, label %if.then, !prof !0
+
+if.then:
+  store i32 %x, ptr %p, align 4
+  br label %if.end
+
+if.end:
+  ret void
+}
+
+define void @p61(i32 %x, ptr %p) {
+; CHECK-LABEL: p61:
+; CHECK:       # %bb.0: # %entry
+; CHECK-NEXT:    testl %edi, %edi
+; CHECK-NEXT:    jne .LBB1_1
+; CHECK-NEXT:  # %bb.2: # %if.end
+; CHECK-NEXT:    retq
+; CHECK-NEXT:  .LBB1_1: # %if.then
+; CHECK-NEXT:    movl %edi, (%rsi)
+; CHECK-NEXT:    retq
+;
+; TH60-LABEL: p61:
+; TH60:       # %bb.0: # %entry
+; TH60-NEXT:    testl %edi, %edi
+; TH60-NEXT:    ds
+; TH60-NEXT:    je .LBB1_2
+; TH60-NEXT:  # %bb.1: # %if.then
+; TH60-NEXT:    movl %edi, (%rsi)
+; TH60-NEXT:  .LBB1_2: # %if.end
+; TH60-NEXT:    retq
+entry:
+  %tobool.not = icmp eq i32 %x, 0
+  br i1 %tobool.not, label %if.end, label %if.then, !prof !1
+
+if.then:
+  store i32 %x, ptr %p, align 4
+  br label %if.end
+
+if.end:
+  ret void
+}
+
+!0 = !{!"branch_weights", i32 51, i32 49}
+!1 = !{!"branch_weights", i32 61, i32 39}