[llvm] [GVN][PGO] Skip GVN if entry BlockFreq is 0 (PR #166336)

[Madhur Amilkanthwar via llvm-commits]

https://github.com/madhur13490 updated https://github.com/llvm/llvm-project/pull/166336

>From 765dff4fca1026390c73a75905b862e586304c90 Mon Sep 17 00:00:00 2001
From: Madhur Amilkanthwar <madhura at nvidia.com>
Date: Thu, 30 Oct 2025 23:16:42 -0700
Subject: [PATCH 1/2] [GVN][PGO] Skip GVN if entry BlockFreq is 0

This patch skips GVN is !prof metadata indicates zero frequency.
---
 llvm/lib/Transforms/Scalar/GVN.cpp            | 12 ++++++
 .../test/Transforms/GVN/skip-gvn-blockfreq.ll | 41 +++++++++++++++++++
 2 files changed, 53 insertions(+)
 create mode 100644 llvm/test/Transforms/GVN/skip-gvn-blockfreq.ll

diff --git a/llvm/lib/Transforms/Scalar/GVN.cpp b/llvm/lib/Transforms/Scalar/GVN.cpp
index 72e1131a54a86..3dee060ee223a 100644
--- a/llvm/lib/Transforms/Scalar/GVN.cpp
+++ b/llvm/lib/Transforms/Scalar/GVN.cpp
@@ -115,6 +115,8 @@ GVNEnableSplitBackedgeInLoadPRE("enable-split-backedge-in-load-pre",
 static cl::opt<bool> GVNEnableMemDep("enable-gvn-memdep", cl::init(true));
 static cl::opt<bool> GVNEnableMemorySSA("enable-gvn-memoryssa",
                                         cl::init(false));
+static cl::opt<bool> GVNSkipZeroEntryCount("gvn-skip-zero-entry-count",
+                                           cl::init(false));
 
 static cl::opt<uint32_t> MaxNumDeps(
     "gvn-max-num-deps", cl::Hidden, cl::init(100),
@@ -894,6 +896,16 @@ PreservedAnalyses GVNPass::run(Function &F, FunctionAnalysisManager &AM) {
     MSSA = &AM.getResult<MemorySSAAnalysis>(F);
   }
   auto &ORE = AM.getResult<OptimizationRemarkEmitterAnalysis>(F);
+  
+  // Skip the pass if function has zero entry count in PGO.
+  // This indicates that the function is never executed according to the profile data.
+  auto EntryCount = F.getEntryCount();
+  if (GVNSkipZeroEntryCount && EntryCount && EntryCount->getCount() == 0) {
+    LLVM_DEBUG(dbgs() << "GVN: Skipping function '" << F.getName()
+                      << "' with zero profile entry count\n");
+    return PreservedAnalyses::all();
+  }
+
   bool Changed = runImpl(F, AC, DT, TLI, AA, MemDep, LI, &ORE,
                          MSSA ? &MSSA->getMSSA() : nullptr);
   if (!Changed)
diff --git a/llvm/test/Transforms/GVN/skip-gvn-blockfreq.ll b/llvm/test/Transforms/GVN/skip-gvn-blockfreq.ll
new file mode 100644
index 0000000000000..c1a33bbda1aa4
--- /dev/null
+++ b/llvm/test/Transforms/GVN/skip-gvn-blockfreq.ll
@@ -0,0 +1,41 @@
+; Test that GVN is skipped when function has zero entry count in PGO
+; RUN: opt -passes='gvn' -gvn-skip-zero-entry-count=true -S < %s | FileCheck %s
+
+; Function with ZERO entry count - GVN should skip this function
+; The redundant computation should remain because GVN doesn't run
+; CHECK-LABEL: @zero_freq_function(
+; CHECK-NEXT: entry:
+; CHECK-NEXT:   %a = add i32 %x, 1
+; CHECK-NEXT:   %b = add i32 %a, 2
+; CHECK-NEXT:   %c = add i32 %a, 2
+; CHECK-NEXT:   %result = add i32 %b, %c
+; CHECK-NEXT:   ret i32 %result
+define i32 @zero_freq_function(i32 %x) !prof !0 {
+entry:
+  %a = add i32 %x, 1
+  %b = add i32 %a, 2
+  %c = add i32 %a, 2    ; Redundant - but GVN should not  optimize due to zero freq
+  %result = add i32 %b, %c
+  ret i32 %result
+}
+
+; Function with NON-ZERO entry count - GVN should run normally
+; The redundant computation should be eliminated by GVN
+; CHECK-LABEL: @nonzero_freq_function(
+; CHECK-NEXT: entry:
+; CHECK-NEXT:   %a = add i32 %x, 1
+; CHECK-NEXT:   %b = add i32 %a, 2
+; CHECK-NEXT:   %result = add i32 %b, %b
+; CHECK-NEXT:   ret i32 %result
+define i32 @nonzero_freq_function(i32 %x) !prof !1 {
+entry:
+  %a = add i32 %x, 1
+  %b = add i32 %a, 2
+  %c = add i32 %a, 2    ; Redundant - GVN optimizes this
+  %result = add i32 %b, %c
+  ret i32 %result
+}
+
+!0 = !{!"function_entry_count", i64 0}      ; Zero frequency
+!1 = !{!"function_entry_count", i64 1000}   ; Non-zero frequency
+

>From 5a4293b767a444799fa3280a354a75485ec7b072 Mon Sep 17 00:00:00 2001
From: Madhur Amilkanthwar <madhura at nvidia.com>
Date: Wed, 5 Nov 2025 23:44:02 -0800
Subject: [PATCH 2/2] fixup! [GVN][PGO] Skip GVN if entry BlockFreq is 0

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