[llvm] [llvm][GVN] Propagate `trunc nuw to i1` equalities (PR #143273)

[Yash Solanki via llvm-commits]

https://github.com/yashnator updated https://github.com/llvm/llvm-project/pull/143273

>From 15db14ca4dc08fc41a6d795c08acae8cb6b4c621 Mon Sep 17 00:00:00 2001
From: Yash Solanki <252yash at gmail.com>
Date: Sat, 7 Jun 2025 20:26:56 +0530
Subject: [PATCH] [GVN] Propagate equality from trunc nuw iN to i1

If the result of `trunc nuw iN` is known to be true/false,
the original value of the truncated integer can be
inferred.

This is folded under GVN's propagateEquality.

Fixes missed optimization noted in #142744
---
 llvm/lib/Transforms/Scalar/GVN.cpp            |  8 ++++++
 .../test/Transforms/GVN/trunc-nuw-equality.ll | 28 +++++++++++++++++++
 2 files changed, 36 insertions(+)
 create mode 100644 llvm/test/Transforms/GVN/trunc-nuw-equality.ll

diff --git a/llvm/lib/Transforms/Scalar/GVN.cpp b/llvm/lib/Transforms/Scalar/GVN.cpp
index 77ca14f88a834..a0eed31fde792 100644
--- a/llvm/lib/Transforms/Scalar/GVN.cpp
+++ b/llvm/lib/Transforms/Scalar/GVN.cpp
@@ -2579,6 +2579,14 @@ bool GVNPass::propagateEquality(Value *LHS, Value *RHS,
 
       continue;
     }
+
+    // Propagate equalities that results from truncation with no unsigned wrap
+    // like (trunc nuw i64 %v to i1) == "true" or (trunc nuw i64 %v to i1) ==
+    // "false"
+    if (match(LHS, m_NUWTrunc(m_Value(A)))) {
+      Worklist.emplace_back(A, ConstantInt::get(A->getType(), IsKnownTrue));
+      continue;
+    }
   }
 
   return Changed;
diff --git a/llvm/test/Transforms/GVN/trunc-nuw-equality.ll b/llvm/test/Transforms/GVN/trunc-nuw-equality.ll
new file mode 100644
index 0000000000000..efe2ca3932031
--- /dev/null
+++ b/llvm/test/Transforms/GVN/trunc-nuw-equality.ll
@@ -0,0 +1,28 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 5
+; RUN: opt -passes=gvn -S < %s | FileCheck %s
+
+define void @test(ptr %p, i64 %v) {
+; CHECK-LABEL: define void @test(
+; CHECK-SAME: ptr [[P:%.*]], i64 [[V:%.*]]) {
+; CHECK-NEXT:  [[ENTRY:.*:]]
+; CHECK-NEXT:    [[TR:%.*]] = trunc nuw i64 [[V]] to i1
+; CHECK-NEXT:    br i1 [[TR]], label %[[RET:.*]], label %[[STORE:.*]]
+; CHECK:       [[STORE]]:
+; CHECK-NEXT:    store i64 0, ptr [[P]], align 4
+; CHECK-NEXT:    ret void
+; CHECK:       [[RET]]:
+; CHECK-NEXT:    store i64 1, ptr [[P]], align 4
+; CHECK-NEXT:    ret void
+;
+entry:
+  %tr = trunc nuw i64 %v to i1
+  br i1 %tr, label %ret, label %store
+
+store:
+  store i64 %v, ptr %p
+  ret void
+
+ret:
+  store i64 %v, ptr %p
+  ret void
+}