[PATCH] D94550: [InstCombine] Fold select -> and/or using impliesPoison

[Nikita Popov via Phabricator via llvm-commits]

nikic created this revision.
nikic added reviewers: aqjune, spatel, lebedev.ri.
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We can fold `a ? b : false` to `a & b` if `is_poison(b)` implies that `is_poison(a)`, at which point we're able to reuse all the usual fold on ands. In particular, this covers the very common case of `icmp X, C && icmp X, C'`.

This currently only has an effect if `-instcombine-unsafe-select-transform=0`.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D94550

Files:
  lib/Transforms/InstCombine/InstCombineSelect.cpp
  test/Transforms/InstCombine/select-and-or.ll


Index: test/Transforms/InstCombine/select-and-or.ll
===================================================================
--- test/Transforms/InstCombine/select-and-or.ll
+++ test/Transforms/InstCombine/select-and-or.ll
@@ -91,7 +91,7 @@
 ; CHECK-LABEL: @logical_or_implies(
 ; CHECK-NEXT:    [[C1:%.*]] = icmp eq i32 [[X:%.*]], 0
 ; CHECK-NEXT:    [[C2:%.*]] = icmp eq i32 [[X]], 42
-; CHECK-NEXT:    [[RES:%.*]] = select i1 [[C1]], i1 true, i1 [[C2]]
+; CHECK-NEXT:    [[RES:%.*]] = or i1 [[C1]], [[C2]]
 ; CHECK-NEXT:    ret i1 [[RES]]
 ;
   %c1 = icmp eq i32 %x, 0
@@ -103,10 +103,7 @@
 ; Will fold after conversion to or.
 define i1 @logical_or_implies_folds(i32 %x) {
 ; CHECK-LABEL: @logical_or_implies_folds(
-; CHECK-NEXT:    [[C1:%.*]] = icmp slt i32 [[X:%.*]], 0
-; CHECK-NEXT:    [[C2:%.*]] = icmp sgt i32 [[X]], -1
-; CHECK-NEXT:    [[RES:%.*]] = select i1 [[C1]], i1 true, i1 [[C2]]
-; CHECK-NEXT:    ret i1 [[RES]]
+; CHECK-NEXT:    ret i1 true
 ;
   %c1 = icmp slt i32 %x, 0
   %c2 = icmp sge i32 %x, 0
@@ -119,7 +116,7 @@
 ; CHECK-LABEL: @logical_and_implies(
 ; CHECK-NEXT:    [[C1:%.*]] = icmp ne i32 [[X:%.*]], 0
 ; CHECK-NEXT:    [[C2:%.*]] = icmp ne i32 [[X]], 42
-; CHECK-NEXT:    [[RES:%.*]] = select i1 [[C1]], i1 [[C2]], i1 false
+; CHECK-NEXT:    [[RES:%.*]] = and i1 [[C1]], [[C2]]
 ; CHECK-NEXT:    ret i1 [[RES]]
 ;
   %c1 = icmp ne i32 %x, 0
@@ -132,9 +129,7 @@
 define i1 @logical_and_implies_folds(i32 %x) {
 ; CHECK-LABEL: @logical_and_implies_folds(
 ; CHECK-NEXT:    [[C1:%.*]] = icmp ugt i32 [[X:%.*]], 42
-; CHECK-NEXT:    [[C2:%.*]] = icmp ne i32 [[X]], 0
-; CHECK-NEXT:    [[RES:%.*]] = select i1 [[C1]], i1 [[C2]], i1 false
-; CHECK-NEXT:    ret i1 [[RES]]
+; CHECK-NEXT:    ret i1 [[C1]]
 ;
   %c1 = icmp ugt i32 %x, 42
   %c2 = icmp ne i32 %x, 0
@@ -153,6 +148,7 @@
 }
 
 ; Noundef on false value allows conversion to or.
+; TODO: impliesPoison doesn't handle this yet.
 define i1 @logical_or_noundef_b(i1 %a, i1 noundef %b) {
 ; CHECK-LABEL: @logical_or_noundef_b(
 ; CHECK-NEXT:    [[RES:%.*]] = select i1 [[A:%.*]], i1 true, i1 [[B:%.*]]
@@ -173,6 +169,7 @@
 }
 
 ; Noundef on false value allows conversion to and.
+; TODO: impliesPoison doesn't handle this yet.
 define i1 @logical_and_noundef_b(i1 %a, i1 noundef %b) {
 ; CHECK-LABEL: @logical_and_noundef_b(
 ; CHECK-NEXT:    [[RES:%.*]] = select i1 [[A:%.*]], i1 [[B:%.*]], i1 false
Index: lib/Transforms/InstCombine/InstCombineSelect.cpp
===================================================================
--- lib/Transforms/InstCombine/InstCombineSelect.cpp
+++ lib/Transforms/InstCombine/InstCombineSelect.cpp
@@ -2572,11 +2572,13 @@
 
   if (SelType->isIntOrIntVectorTy(1) &&
       TrueVal->getType() == CondVal->getType()) {
-    if (EnableUnsafeSelectTransform && match(TrueVal, m_One())) {
+    if (match(TrueVal, m_One()) &&
+        (EnableUnsafeSelectTransform || impliesPoison(FalseVal, CondVal))) {
       // Change: A = select B, true, C --> A = or B, C
       return BinaryOperator::CreateOr(CondVal, FalseVal);
     }
-    if (EnableUnsafeSelectTransform && match(FalseVal, m_Zero())) {
+    if (match(FalseVal, m_Zero()) &&
+        (EnableUnsafeSelectTransform || impliesPoison(TrueVal, CondVal))) {
       // Change: A = select B, C, false --> A = and B, C
       return BinaryOperator::CreateAnd(CondVal, TrueVal);
     }


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