[llvm] [AMDGPU] Eliminate likely-spurious execz checks (PR #117567)

[Matt Arsenault via llvm-commits]

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+//===- AMDGPUAnnotateVaryingBranchWeights.cpp -----------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+// Estimate if conditional branches for which SIAnnotateControlFlow introduced
+// amdgcn_if or amdgcn_else intrinsics are likely to have different outcomes for
+// the threads of each wavefront. If that is the case, BranchWeight metadata is
+// added to signal that "then" and "else" blocks are both likely to be executed.
+// This may introduce branch weights that would be self-contradictory in a
+// non-SIMT setting.
+//
+// A consequence of this is that SIPreEmitPeephole is more likely to eliminate
+// s_cbranch_execz instructions that were introduced to skip these blocks when
+// no thread in the wavefront is active for them.
+//
+// Should only run after SIAnnotateControlFlow.
+//===----------------------------------------------------------------------===//
+
+#include "AMDGPU.h"
+#include "AMDGPUTargetMachine.h"
+#include "GCNSubtarget.h"
+#include "llvm/Analysis/LazyValueInfo.h"
+#include "llvm/Analysis/TargetTransformInfo.h"
+#include "llvm/Analysis/ValueTracking.h"
+#include "llvm/CodeGen/TargetPassConfig.h"
+#include "llvm/IR/Analysis.h"
+#include "llvm/IR/Instructions.h"
+#include "llvm/IR/IntrinsicsAMDGPU.h"
+#include "llvm/IR/PatternMatch.h"
+#include "llvm/IR/ProfDataUtils.h"
+#include "llvm/InitializePasses.h"
+#include "llvm/Support/Casting.h"
+#include "llvm/Target/TargetMachine.h"
+
+using namespace llvm;
+
+#define DEBUG_TYPE "amdgpu-annotate-varying-branch-weights"
+
+namespace {
+
+class AMDGPUAnnotateVaryingBranchWeightsImpl {
+public:
+  AMDGPUAnnotateVaryingBranchWeightsImpl() = delete;
+  AMDGPUAnnotateVaryingBranchWeightsImpl(const GCNSubtarget &ST,
+                                         const TargetTransformInfo &TTI)
+      : ST(ST), TTI(TTI) {
+    // Determine weights that signal that a branch is very likely to be
+    // predicted correctly, i.e., whose ratio exceeds
+    // TTI.getPredictableBranchThreshold().
+    auto BranchProbThreshold = TTI.getPredictableBranchThreshold();
+    LikelyWeight = BranchProbThreshold.getNumerator();
+    UnlikelyWeight = BranchProbThreshold.getDenominator() - LikelyWeight;
+    if (UnlikelyWeight > 0)
+      --UnlikelyWeight;
+  }
+
+  bool run(Function &F);
+
+private:
+  const GCNSubtarget &ST;
+  const TargetTransformInfo &TTI;
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arsenm wrote:

GCNTTI?

https://github.com/llvm/llvm-project/pull/117567