[ValueTracking] Handle trunc to i1 as condition in dominating condition. #126414

andjo403 · 2025-02-09T11:13:00Z

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

llvmbot · 2025-02-09T11:13:32Z

@llvm/pr-subscribers-llvm-transforms

@llvm/pr-subscribers-llvm-analysis

Author: Andreas Jonson (andjo403)

Changes

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

Full diff: https://github.com/llvm/llvm-project/pull/126414.diff

2 Files Affected:

(modified) llvm/lib/Analysis/ValueTracking.cpp (+19)
(modified) llvm/test/Transforms/InstCombine/known-bits.ll (+10-20)

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

nikic · 2025-02-09T11:29:27Z

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;


Not in a br cond will be canonicalized away by swapping successors. Shouldn't need this.

notice at least some regressions in opt-benchmark when I removed it,
one example I think is this pattern:

%630 = trunc i8 %629 to i1 %not. = xor i1 %630, true %.0215.shrunk. = or i1 %spec.select472, %not. br i1 %.0215.shrunk., label %632, label %.thread

so I shall maybe try to make a test that cover this case?

added the @test_not_trunc_cond_and that do not pass without this.

Can you please split this off into a followup change? I think it makes sense to handle it, but if we do so, we should do it generically, not for trunc conditions in particular.

goldsteinn · 2025-02-09T16:47:49Z

What are your proofs proving? The tgt and src seem to be duplicated...

goldsteinn · 2025-02-09T16:49:55Z

llvm/lib/Analysis/ValueTracking.cpp

+      DstKnown.setAllOnes();
+    }
+    if (cast<TruncInst>(Cond)->hasNoUnsignedWrap())
+      Known = Known.unionWith(DstKnown.zext(Known.getBitWidth()));


you can sext if you have trunc nsw

avoided sext as it was not handled i #125414 seems like it is uncommon to have sext for trunc to i1 only found 3 in llvm-opt-benchmark

andjo403 · 2025-02-09T17:56:19Z

fixed the proofs copy past is good when looking at what is copied.

dtcxzyw · 2025-02-10T06:22:20Z

llvm/lib/Analysis/ValueTracking.cpp

@@ -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)),


Is !IsAssume used to avoid issues with ephemeral values? If so, please add some comments here.

partly, for the not #126423 (comment) it is the avoid issues with ephemeral values.
but also to avoid to add the same value again as truncated value of V is already added for assume above as the AddAffected Looks through trunc.
will add comment

andjo403 · 2025-02-10T18:37:23Z

rebased on top of #126423 so ready for review again

nikic

LGTM

…on. (llvm#126414) proof: https://alive2.llvm.org/ce/z/gALGmv

andjo403 requested review from nikic, dtcxzyw and goldsteinn February 9, 2025 11:13

llvmbot added llvm:instcombine llvm:analysis llvm:transforms labels Feb 9, 2025

andjo403 mentioned this pull request Feb 9, 2025

[ValueTracking] Handle assume(trunc x to i1) in ComputeKnownBits #118406

Merged

nikic mentioned this pull request Feb 9, 2025

Task submission dtcxzyw/llvm-opt-benchmark#1312

Open

dtcxzyw mentioned this pull request Feb 9, 2025

pre-commit: PR126414 dtcxzyw/llvm-opt-benchmark#2109

Closed

nikic reviewed Feb 9, 2025

View reviewed changes

andjo403 force-pushed the truncCondInDominatingCond branch from a684f10 to d148f62 Compare February 9, 2025 12:59

goldsteinn reviewed Feb 9, 2025

View reviewed changes

andjo403 mentioned this pull request Feb 9, 2025

[ValueTracking] Handle not in dominating condition. #126423

Merged

dtcxzyw reviewed Feb 10, 2025

View reviewed changes

[ValueTracking] Handle trunc to i1 as condition in dominating condition.

167513c

andjo403 force-pushed the truncCondInDominatingCond branch from d148f62 to 167513c Compare February 10, 2025 18:33

nikic approved these changes Feb 11, 2025

View reviewed changes

andjo403 merged commit cf87eb9 into llvm:main Feb 11, 2025
8 checks passed

Icohedron pushed a commit to Icohedron/llvm-project that referenced this pull request Feb 11, 2025

[ValueTracking] Handle trunc to i1 as condition in dominating conditi…

df2fe27

…on. (llvm#126414) proof: https://alive2.llvm.org/ce/z/gALGmv

flovent pushed a commit to flovent/llvm-project that referenced this pull request Feb 13, 2025

[ValueTracking] Handle trunc to i1 as condition in dominating conditi…

044d916

…on. (llvm#126414) proof: https://alive2.llvm.org/ce/z/gALGmv

joaosaffran pushed a commit to joaosaffran/llvm-project that referenced this pull request Feb 14, 2025

[ValueTracking] Handle trunc to i1 as condition in dominating conditi…

3f8e4fd

…on. (llvm#126414) proof: https://alive2.llvm.org/ce/z/gALGmv

andjo403 deleted the truncCondInDominatingCond branch February 16, 2025 12:14

sivan-shani pushed a commit to sivan-shani/llvm-project that referenced this pull request Feb 24, 2025

[ValueTracking] Handle trunc to i1 as condition in dominating conditi…

a209abb

…on. (llvm#126414) proof: https://alive2.llvm.org/ce/z/gALGmv

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