[llvm] [ValueTracking] Handle trunc to i1 as condition in dominating condition. (PR #126414)

[Andreas Jonson via llvm-commits]

https://github.com/andjo403 created https://github.com/llvm/llvm-project/pull/126414

proof: https://alive2.llvm.org/ce/z/j5fZgV

>From a684f102d499c252cf653dbf9a4b7f4caa5646b9 Mon Sep 17 00:00:00 2001
From: Andreas Jonson <andjo403 at hotmail.com>
Date: Wed, 5 Feb 2025 21:29:03 +0100
Subject: [PATCH] [ValueTracking] Handle trunc to i1 as condition in dominating
 condition.

---
 llvm/lib/Analysis/ValueTracking.cpp           | 19 ++++++++++++
 .../test/Transforms/InstCombine/known-bits.ll | 30 +++++++------------
 2 files changed, 29 insertions(+), 20 deletions(-)

diff --git a/llvm/lib/Analysis/ValueTracking.cpp b/llvm/lib/Analysis/ValueTracking.cpp
index 8a9ad55366ee703..40fae7ca95d774b 100644
--- a/llvm/lib/Analysis/ValueTracking.cpp
+++ b/llvm/lib/Analysis/ValueTracking.cpp
@@ -801,6 +801,22 @@ static void computeKnownBitsFromCond(const Value *V, Value *Cond,
 
   if (auto *Cmp = dyn_cast<ICmpInst>(Cond))
     computeKnownBitsFromICmpCond(V, Cmp, Known, SQ, Invert);
+
+  if (match(Cond, m_Not(m_Value(Cond))))
+    Invert = !Invert;
+
+  if (match(Cond, m_Trunc(m_Specific(V)))) {
+    KnownBits DstKnown(1);
+    if (Invert) {
+      DstKnown.setAllZero();
+    } else {
+      DstKnown.setAllOnes();
+    }
+    if (cast<TruncInst>(Cond)->hasNoUnsignedWrap())
+      Known = Known.unionWith(DstKnown.zext(Known.getBitWidth()));
+    else
+      Known = Known.unionWith(DstKnown.anyext(Known.getBitWidth()));
+  }
 }
 
 void llvm::computeKnownBitsFromContext(const Value *V, KnownBits &Known,
@@ -10272,6 +10288,9 @@ void llvm::findValuesAffectedByCondition(
                                                            m_Value()))) {
       // Handle patterns that computeKnownFPClass() support.
       AddAffected(A);
+    } else if (!IsAssume && match(V, m_CombineOr(m_Trunc(m_Value(X)),
+                                                 m_Not(m_Trunc(m_Value(X)))))) {
+      AddAffected(X);
     }
   }
 }
diff --git a/llvm/test/Transforms/InstCombine/known-bits.ll b/llvm/test/Transforms/InstCombine/known-bits.ll
index a3872fefecf3b31..b47c3de21f9feaf 100644
--- a/llvm/test/Transforms/InstCombine/known-bits.ll
+++ b/llvm/test/Transforms/InstCombine/known-bits.ll
@@ -2167,11 +2167,9 @@ define i8 @test_trunc_and_1(i8 %a) {
 ; CHECK-NEXT:    [[CAST:%.*]] = trunc i8 [[A:%.*]] to i1
 ; CHECK-NEXT:    br i1 [[CAST]], label [[IF_THEN:%.*]], label [[IF_ELSE:%.*]]
 ; CHECK:       if.then:
-; CHECK-NEXT:    [[B:%.*]] = and i8 [[A]], 1
-; CHECK-NEXT:    ret i8 [[B]]
+; CHECK-NEXT:    ret i8 1
 ; CHECK:       if.else:
-; CHECK-NEXT:    [[C:%.*]] = and i8 [[A]], 1
-; CHECK-NEXT:    ret i8 [[C]]
+; CHECK-NEXT:    ret i8 0
 ;
 entry:
   %cast = trunc i8 %a to i1
@@ -2192,11 +2190,9 @@ define i8 @test_not_trunc_and_1(i8 %a) {
 ; CHECK-NEXT:    [[CAST:%.*]] = trunc i8 [[A:%.*]] to i1
 ; CHECK-NEXT:    br i1 [[CAST]], label [[IF_ELSE:%.*]], label [[IF_THEN:%.*]]
 ; CHECK:       if.then:
-; CHECK-NEXT:    [[B:%.*]] = and i8 [[A]], 1
-; CHECK-NEXT:    ret i8 [[B]]
+; CHECK-NEXT:    ret i8 0
 ; CHECK:       if.else:
-; CHECK-NEXT:    [[C:%.*]] = and i8 [[A]], 1
-; CHECK-NEXT:    ret i8 [[C]]
+; CHECK-NEXT:    ret i8 1
 ;
 entry:
   %cast = trunc i8 %a to i1
@@ -2243,11 +2239,9 @@ define i8 @test_trunc_nuw_and_1(i8 %a) {
 ; CHECK-NEXT:    [[CAST:%.*]] = trunc nuw i8 [[A:%.*]] to i1
 ; CHECK-NEXT:    br i1 [[CAST]], label [[IF_ELSE:%.*]], label [[IF_THEN:%.*]]
 ; CHECK:       if.then:
-; CHECK-NEXT:    [[B:%.*]] = and i8 [[A]], 1
-; CHECK-NEXT:    ret i8 [[B]]
+; CHECK-NEXT:    ret i8 0
 ; CHECK:       if.else:
-; CHECK-NEXT:    [[C:%.*]] = and i8 [[A]], 1
-; CHECK-NEXT:    ret i8 [[C]]
+; CHECK-NEXT:    ret i8 1
 ;
 entry:
   %cast = trunc nuw i8 %a to i1
@@ -2268,11 +2262,9 @@ define i8 @test_trunc_nuw_or_2(i8 %a) {
 ; CHECK-NEXT:    [[CAST:%.*]] = trunc nuw i8 [[A:%.*]] to i1
 ; CHECK-NEXT:    br i1 [[CAST]], label [[IF_ELSE:%.*]], label [[IF_THEN:%.*]]
 ; CHECK:       if.then:
-; CHECK-NEXT:    [[B:%.*]] = or i8 [[A]], 2
-; CHECK-NEXT:    ret i8 [[B]]
+; CHECK-NEXT:    ret i8 2
 ; CHECK:       if.else:
-; CHECK-NEXT:    [[C:%.*]] = or i8 [[A]], 2
-; CHECK-NEXT:    ret i8 [[C]]
+; CHECK-NEXT:    ret i8 3
 ;
 entry:
   %cast = trunc nuw i8 %a to i1
@@ -2293,11 +2285,9 @@ define i8 @test_not_trunc_nuw_and_1(i8 %a) {
 ; CHECK-NEXT:    [[CAST:%.*]] = trunc nuw i8 [[A:%.*]] to i1
 ; CHECK-NEXT:    br i1 [[CAST]], label [[IF_ELSE:%.*]], label [[IF_THEN:%.*]]
 ; CHECK:       if.then:
-; CHECK-NEXT:    [[B:%.*]] = and i8 [[A]], 1
-; CHECK-NEXT:    ret i8 [[B]]
+; CHECK-NEXT:    ret i8 0
 ; CHECK:       if.else:
-; CHECK-NEXT:    [[C:%.*]] = and i8 [[A]], 1
-; CHECK-NEXT:    ret i8 [[C]]
+; CHECK-NEXT:    ret i8 1
 ;
 entry:
   %cast = trunc nuw i8 %a to i1