[PATCH] D134456: [PGO] Consider parent context when weighing branches with likelyhood.

[Anton Bikineev via Phabricator via cfe-commits]

This revision was automatically updated to reflect the committed changes.
Closed by commit rG7b85e765000d: [PGO] Consider parent context when weighing branches with likelyhood. (authored by AntonBikineev).

Changed prior to commit:
  https://reviews.llvm.org/D134456?vs=462250&id=466313#toc

Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D134456/new/

https://reviews.llvm.org/D134456

Files:
  clang/lib/CodeGen/CGStmt.cpp
  clang/test/Profile/Inputs/cxx-never-executed-branch.proftext
  clang/test/Profile/cxx-never-executed-branch.cpp


Index: clang/test/Profile/cxx-never-executed-branch.cpp
===================================================================
--- /dev/null
+++ clang/test/Profile/cxx-never-executed-branch.cpp
@@ -0,0 +1,32 @@
+// Test that clang doesn't emit llvm.expect when the counter is 0
+
+// RUN: llvm-profdata merge %S/Inputs/cxx-never-executed-branch.proftext -o %t.profdata
+// RUN: %clang_cc1 -std=c++20 %s -triple %itanium_abi_triple -O2 -o - -emit-llvm -fprofile-instrument-use-path=%t.profdata -disable-llvm-passes | FileCheck %s
+
+int rand();
+
+// CHECK: define {{.*}}@_Z13is_in_profilev
+// CHECK-NOT: call {{.*}}@llvm.expect
+
+int is_in_profile() {
+  int rando = rand();
+  int x = 0;
+  if (rando == 0) [[likely]]
+    x = 2;
+  else
+    x = 3;
+  return x;
+}
+
+// CHECK: define {{.*}}@_Z17is_not_in_profilev
+// CHECK: call {{.*}}@llvm.expect
+
+int is_not_in_profile() {
+  int rando = rand();
+  int x = 0;
+  if (rando == 0) [[likely]]
+    x = 2;
+  else
+    x = 3;
+  return x;
+}
Index: clang/test/Profile/Inputs/cxx-never-executed-branch.proftext
===================================================================
--- /dev/null
+++ clang/test/Profile/Inputs/cxx-never-executed-branch.proftext
@@ -0,0 +1,13 @@
+_Z13is_in_profilev
+0x000000029f493458
+2
+20000
+# The branch is never executed.
+0
+
+_Z17is_not_in_profilev
+0x000000029f493458
+2
+# The function was never executed
+0
+0
Index: clang/lib/CodeGen/CGStmt.cpp
===================================================================
--- clang/lib/CodeGen/CGStmt.cpp
+++ clang/lib/CodeGen/CGStmt.cpp
@@ -815,11 +815,20 @@
   // Prefer the PGO based weights over the likelihood attribute.
   // When the build isn't optimized the metadata isn't used, so don't generate
   // it.
+  // Also, differentiate between disabled PGO and a never executed branch with
+  // PGO. Assuming PGO is in use:
+  // - we want to ignore the [[likely]] attribute if the branch is never
+  // executed,
+  // - assuming the profile is poor, preserving the attribute may still be
+  // beneficial.
+  // As an approximation, preserve the attribute only if both the branch and the
+  // parent context were not executed.
   Stmt::Likelihood LH = Stmt::LH_None;
-  uint64_t Count = getProfileCount(S.getThen());
-  if (!Count && CGM.getCodeGenOpts().OptimizationLevel)
+  uint64_t ThenCount = getProfileCount(S.getThen());
+  if (!ThenCount && !getCurrentProfileCount() &&
+      CGM.getCodeGenOpts().OptimizationLevel)
     LH = Stmt::getLikelihood(S.getThen(), S.getElse());
-  EmitBranchOnBoolExpr(S.getCond(), ThenBlock, ElseBlock, Count, LH);
+  EmitBranchOnBoolExpr(S.getCond(), ThenBlock, ElseBlock, ThenCount, LH);
 
   // Emit the 'then' code.
   EmitBlock(ThenBlock);


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