[InstCombine] Fold select of symmetric selects #99245

tgymnich · 2024-07-16T21:32:49Z

fixes #98800

Fold patterns like:
select c2 (select c1 a b) (select c1 b a)
into:
select (xor c1 c2) b a

Alive2 proofs:
https://alive2.llvm.org/ce/z/4QAm4K
https://alive2.llvm.org/ce/z/vTVRnC

llvmbot · 2024-07-16T21:35:23Z

@llvm/pr-subscribers-llvm-transforms

Author: Tim Gymnich (tgymnich)

Changes

fixes #98800

Fold patterns like:
select c2 (select c1 a b) (select c1 b a)
into:
select (xor c1 c2) b a

Full diff: https://github.com/llvm/llvm-project/pull/99245.diff

2 Files Affected:

(modified) llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp (+25)
(added) llvm/test/Transforms/InstCombine/select-of-symmetric-selects.ll (+92)

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp b/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
index 394dfca262e13..67f8986abef36 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
@@ -3012,6 +3012,28 @@ struct DecomposedSelect {
 };
 } // namespace
 
+/// Folds patterns like:
+///   select c2 (select c1 a b) (select c1 b a)
+/// into:
+///   select (xor c1 c2) b a
+static Instruction *
+foldSelectOfSymmetricSelect(SelectInst &OuterSelVal,
+                            InstCombiner::BuilderTy &Builder) {
+
+  DecomposedSelect OuterSel, InnerSel;
+  if (!match(&OuterSelVal, m_Select(m_Value(OuterSel.Cond),
+                                    m_Select(m_Value(InnerSel.Cond),
+                                             m_Value(InnerSel.TrueVal),
+                                             m_Value(InnerSel.FalseVal)),
+                                    m_Select(m_Deferred(InnerSel.Cond),
+                                             m_Deferred(InnerSel.FalseVal),
+                                             m_Deferred(InnerSel.TrueVal)))))
+    return nullptr;
+
+  Value *Xor = Builder.CreateXor(InnerSel.Cond, OuterSel.Cond);
+  return SelectInst::Create(Xor, InnerSel.FalseVal, InnerSel.TrueVal);
+}
+
 /// Look for patterns like
 ///   %outer.cond = select i1 %inner.cond, i1 %alt.cond, i1 false
 ///   %inner.sel = select i1 %inner.cond, i8 %inner.sel.t, i8 %inner.sel.f
@@ -3987,6 +4009,9 @@ Instruction *InstCombinerImpl::visitSelectInst(SelectInst &SI) {
     }
   }
 
+  if (Instruction *I = foldSelectOfSymmetricSelect(SI, Builder))
+    return I;
+
   if (Instruction *I = foldNestedSelects(SI, Builder))
     return I;
 
diff --git a/llvm/test/Transforms/InstCombine/select-of-symmetric-selects.ll b/llvm/test/Transforms/InstCombine/select-of-symmetric-selects.ll
new file mode 100644
index 0000000000000..9391f841482ae
--- /dev/null
+++ b/llvm/test/Transforms/InstCombine/select-of-symmetric-selects.ll
@@ -0,0 +1,92 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt < %s -passes=instcombine -S | FileCheck %s
+
+define i32 @select_of_symmetric_selects(i32 %a, i32 %b, i1 %c1, i1 %c2) {
+; CHECK-LABEL: @select_of_symmetric_selects(
+; CHECK-NEXT:    [[TMP1:%.*]] = xor i1 [[C1:%.*]], [[C2:%.*]]
+; CHECK-NEXT:    [[RET:%.*]] = select i1 [[TMP1]], i32 [[B:%.*]], i32 [[A:%.*]]
+; CHECK-NEXT:    ret i32 [[RET]]
+;
+  %sel1 = select i1 %c1, i32 %a, i32 %b
+  %sel2 = select i1 %c1, i32 %b, i32 %a
+  %ret = select i1 %c2, i32 %sel1, i32 %sel2
+  ret i32 %ret
+}
+
+define i32 @select_of_symmetric_selects_negative1(i32 %a, i32 %b, i1 %c1, i1 %c2) {
+; CHECK-LABEL: @select_of_symmetric_selects_negative1(
+; CHECK-NEXT:    [[SEL1:%.*]] = select i1 [[C1:%.*]], i32 [[A:%.*]], i32 [[B:%.*]]
+; CHECK-NEXT:    [[RET:%.*]] = select i1 [[C2:%.*]], i32 [[SEL1]], i32 [[A]]
+; CHECK-NEXT:    ret i32 [[RET]]
+;
+  %sel1 = select i1 %c1, i32 %a, i32 %b
+  %sel2 = select i1 %c2, i32 %b, i32 %a
+  %ret = select i1 %c2, i32 %sel1, i32 %sel2
+  ret i32 %ret
+}
+
+define i32 @select_of_symmetric_selects_negative2(i32 %a, i32 %b, i32 %c, i1 %c1, i1 %c2) {
+; CHECK-LABEL: @select_of_symmetric_selects_negative2(
+; CHECK-NEXT:    [[SEL1:%.*]] = select i1 [[C1:%.*]], i32 [[A:%.*]], i32 [[B:%.*]]
+; CHECK-NEXT:    [[SEL2:%.*]] = select i1 [[C1]], i32 [[B]], i32 [[C:%.*]]
+; CHECK-NEXT:    [[RET:%.*]] = select i1 [[C2:%.*]], i32 [[SEL1]], i32 [[SEL2]]
+; CHECK-NEXT:    ret i32 [[RET]]
+;
+  %sel1 = select i1 %c1, i32 %a, i32 %b
+  %sel2 = select i1 %c1, i32 %b, i32 %c
+  %ret = select i1 %c2, i32 %sel1, i32 %sel2
+  ret i32 %ret
+}
+
+declare void @use(i32)
+
+define i32 @select_of_symmetric_selects_multi_use(i32 %a, i32 %b, i1 %c1, i1 %c2) {
+; CHECK-LABEL: @select_of_symmetric_selects_multi_use(
+; CHECK-NEXT:    [[SEL2:%.*]] = select i1 [[C1:%.*]], i32 [[B:%.*]], i32 [[A:%.*]]
+; CHECK-NEXT:    call void @use(i32 [[SEL2]])
+; CHECK-NEXT:    [[TMP1:%.*]] = xor i1 [[C1]], [[C2:%.*]]
+; CHECK-NEXT:    [[RET:%.*]] = select i1 [[TMP1]], i32 [[B]], i32 [[A]]
+; CHECK-NEXT:    ret i32 [[RET]]
+;
+  %sel1 = select i1 %c1, i32 %a, i32 %b
+  %sel2 = select i1 %c1, i32 %b, i32 %a
+  call void @use(i32 %sel2)
+  %ret = select i1 %c2, i32 %sel1, i32 %sel2
+  ret i32 %ret
+}
+
+define i32 @select_of_symmetric_selects_commuted(i32 %a, i32 %b, i1 %c1, i1 %c2) {
+; CHECK-LABEL: @select_of_symmetric_selects_commuted(
+; CHECK-NEXT:    [[TMP1:%.*]] = xor i1 [[C1:%.*]], [[C2:%.*]]
+; CHECK-NEXT:    [[RET:%.*]] = select i1 [[TMP1]], i32 [[A:%.*]], i32 [[B:%.*]]
+; CHECK-NEXT:    ret i32 [[RET]]
+;
+  %sel1 = select i1 %c1, i32 %a, i32 %b
+  %sel2 = select i1 %c1, i32 %b, i32 %a
+  %ret = select i1 %c2, i32 %sel2, i32 %sel1
+  ret i32 %ret
+}
+
+define <4 x i32> @select_of_symmetric_selects_vector1(<4 x i32> %a, <4 x i32> %b, i1 %c1, i1 %c2) {
+; CHECK-LABEL: @select_of_symmetric_selects_vector1(
+; CHECK-NEXT:    [[TMP1:%.*]] = xor i1 [[C1:%.*]], [[C2:%.*]]
+; CHECK-NEXT:    [[RET:%.*]] = select i1 [[TMP1]], <4 x i32> [[A:%.*]], <4 x i32> [[B:%.*]]
+; CHECK-NEXT:    ret <4 x i32> [[RET]]
+;
+  %sel1 = select i1 %c1, <4 x i32> %a, <4 x i32> %b
+  %sel2 = select i1 %c1, <4 x i32> %b, <4 x i32> %a
+  %ret = select i1 %c2, <4 x i32> %sel2, <4 x i32> %sel1
+  ret <4 x i32> %ret
+}
+
+define <4 x i32> @select_of_symmetric_selects_vector2(<4 x i32> %a, <4 x i32> %b, <4 x i1> %c1, <4 x i1> %c2) {
+; CHECK-LABEL: @select_of_symmetric_selects_vector2(
+; CHECK-NEXT:    [[TMP1:%.*]] = xor <4 x i1> [[C1:%.*]], [[C2:%.*]]
+; CHECK-NEXT:    [[RET:%.*]] = select <4 x i1> [[TMP1]], <4 x i32> [[A:%.*]], <4 x i32> [[B:%.*]]
+; CHECK-NEXT:    ret <4 x i32> [[RET]]
+;
+  %sel1 = select <4 x i1> %c1, <4 x i32> %a, <4 x i32> %b
+  %sel2 = select <4 x i1> %c1, <4 x i32> %b, <4 x i32> %a
+  %ret = select <4 x i1> %c2, <4 x i32> %sel2, <4 x i32> %sel1
+  ret <4 x i32> %ret
+}

goldsteinn · 2024-07-17T06:52:50Z

llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp

+                                             m_Value(InnerSel.FalseVal)),
+                                    m_Select(m_Deferred(InnerSel.Cond),
+                                             m_Deferred(InnerSel.FalseVal),
+                                             m_Deferred(InnerSel.TrueVal)))))


Any value in handling select c2 (select c1 a b), (select ~c1, a, b) with isKnownInversion?

I guess not. See the fuzzy matching results in dtcxzyw/llvm-opt-benchmark#976 and dtcxzyw/llvm-opt-benchmark#977.

dtcxzyw · 2024-07-17T07:16:38Z

Can you move the tests from #99244 into this PR?
See https://llvm.org/docs/InstCombineContributorGuide.html#precommit-tests.

llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp

llvm/test/Transforms/InstCombine/select-of-symmetric-selects.ll

Alive2 proofs: https://alive2.llvm.org/ce/z/4QAm4K https://alive2.llvm.org/ce/z/vTVRnC

llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp

tgymnich · 2024-07-17T12:53:45Z

Would be nice if we could also proof the splat case:

define <2 x i4> @src(<2 x i4> %a, <2 x i4> %b, <2 x i1> %c1, i1 %c2) {
#0:
  %sel1 = select <2 x i1> %c1, <2 x i4> %a, <2 x i4> %b
  %sel2 = select <2 x i1> %c1, <2 x i4> %b, <2 x i4> %a
  %ret = select i1 %c2, <2 x i4> %sel1, <2 x i4> %sel2
  ret <2 x i4> %ret
}
=>
define <2 x i4> @tgt(<2 x i4> %a, <2 x i4> %b, <2 x i1> %c1, i1 %c2) {
#0:
  %splat = insertelement <2 x i1> poison, i1 %c2, i32 0
  %splat1 = insertelement <2 x i1> %splat, i1 %c2, i32 1
  %xor = xor <2 x i1> %c1, %splat1
  %ret = select <2 x i1> %xor, <2 x i4> %b, <2 x i4> %a
  ret <2 x i4> %ret
}
Transformation doesn't verify!

ERROR: Target is more poisonous than source

Example:
<2 x i4> %a = < #x0 (0)	[based on undef value], #x0 (0)	[based on undef value] >
<2 x i4> %b = < poison, poison >
<2 x i1> %c1 = < #x0 (0), #x1 (1) >
i1 %c2 = undef

Source:
<2 x i4> %sel1 = < poison, #x0 (0) >
<2 x i4> %sel2 = < #x0 (0), poison >
<2 x i4> %ret = < poison, poison >

Target:
<2 x i1> %splat = < #x1 (1), poison >
<2 x i1> %splat1 = < #x1 (1), #x0 (0) >
<2 x i1> %xor = < #x1 (1), #x1 (1) >
<2 x i4> %ret = < poison, poison >
Source value: < poison, poison >
Target value: < poison, poison >

Pure vector case verifies just fine: https://alive2.llvm.org/ce/z/vTVRnC

PR Link: llvm/llvm-project#99245

dtcxzyw

LGTM. Thank you!
Please attach the alive2 proof to the PR description.

tgymnich · 2024-07-17T14:04:02Z

You are welcome.

done!

dtcxzyw · 2024-07-17T14:07:47Z

@tgymnich Do you have commit access?

tgymnich · 2024-07-17T14:08:46Z

@dtcxzyw I don't have commit access

sftlbcn · 2024-07-17T14:10:19Z

would it be ok if only one of the inner selects is oneuse?

nikic

LGTM

tgymnich · 2024-07-17T14:19:48Z

@sftlbcn Not on x86_64 and aarch64 https://godbolt.org/z/na7f9nMKT

Summary: fixes #98800 Fold patterns like: select c2 (select c1 a b) (select c1 b a) into: select (xor c1 c2) b a Alive2 proofs: https://alive2.llvm.org/ce/z/4QAm4K https://alive2.llvm.org/ce/z/vTVRnC Test Plan: Reviewers: Subscribers: Tasks: Tags: Differential Revision: https://phabricator.intern.facebook.com/D60250948

tgymnich changed the title ~~[InstCombine] Select~~ [InstCombine] Select of Symmetric Selects Jul 16, 2024

tgymnich mentioned this pull request Jul 16, 2024

[InstCombine][NFC] Pre-Commit Tests for Select of Symmetric Selects #99244

Closed

tgymnich marked this pull request as ready for review July 16, 2024 21:34

tgymnich requested a review from nikic as a code owner July 16, 2024 21:34

llvmbot added the llvm:transforms label Jul 16, 2024

goldsteinn requested a review from dtcxzyw July 17, 2024 06:51

goldsteinn reviewed Jul 17, 2024

View reviewed changes

dtcxzyw requested changes Jul 17, 2024

View reviewed changes

llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp Outdated Show resolved Hide resolved

nikic reviewed Jul 17, 2024

View reviewed changes

llvm/test/Transforms/InstCombine/select-of-symmetric-selects.ll Show resolved Hide resolved

This was referenced Jul 17, 2024

Grep pr99245 dtcxzyw/llvm-opt-benchmark#976

Closed

Grep pr99245 (Constant) dtcxzyw/llvm-opt-benchmark#977

Closed

tgymnich added 3 commits July 17, 2024 14:45

add tests

d506897

[InstCombine] Add fold for select of symmetric selects

a53ff1c

Alive2 proofs: https://alive2.llvm.org/ce/z/4QAm4K https://alive2.llvm.org/ce/z/vTVRnC

update tests

94abd42

tgymnich force-pushed the sel-of-sym-sels branch from f68723b to 94abd42 Compare July 17, 2024 12:46

dtcxzyw reviewed Jul 17, 2024

View reviewed changes

llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp Outdated Show resolved Hide resolved

simplify DecomposedSelect

714fad2

dtcxzyw added a commit to dtcxzyw/llvm-opt-benchmark that referenced this pull request Jul 17, 2024

pre-commit: test PR99245

847ef64

PR Link: llvm/llvm-project#99245

dtcxzyw mentioned this pull request Jul 17, 2024

pre-commit: test PR99245 dtcxzyw/llvm-opt-benchmark#980

Closed

dtcxzyw approved these changes Jul 17, 2024

View reviewed changes

nikic approved these changes Jul 17, 2024

View reviewed changes

dtcxzyw changed the title ~~[InstCombine] Select of Symmetric Selects~~ [InstCombine] Fold select of symmetric selects Jul 17, 2024

dtcxzyw merged commit c034c44 into llvm:main Jul 17, 2024
5 of 6 checks passed

tgymnich deleted the sel-of-sym-sels branch October 9, 2024 19:42

Provide feedback

Saved searches

Use saved searches to filter your results more quickly

Uh oh!

[InstCombine] Fold select of symmetric selects #99245

[InstCombine] Fold select of symmetric selects #99245

Uh oh!

tgymnich commented Jul 16, 2024 •

edited

Loading

Uh oh!

llvmbot commented Jul 16, 2024

Uh oh!

goldsteinn Jul 17, 2024

Uh oh!

dtcxzyw Jul 17, 2024

Uh oh!

dtcxzyw commented Jul 17, 2024

Uh oh!

Uh oh!

Uh oh!

Uh oh!

tgymnich commented Jul 17, 2024 •

edited

Loading

Uh oh!

dtcxzyw left a comment

Uh oh!

tgymnich commented Jul 17, 2024

Uh oh!

dtcxzyw commented Jul 17, 2024

Uh oh!

tgymnich commented Jul 17, 2024 •

edited

Loading

Uh oh!

sftlbcn commented Jul 17, 2024

Uh oh!

nikic left a comment

Uh oh!

tgymnich commented Jul 17, 2024

Uh oh!

Uh oh!

Uh oh!

[InstCombine] Fold select of symmetric selects #99245

[InstCombine] Fold select of symmetric selects #99245

Uh oh!

Conversation

tgymnich commented Jul 16, 2024 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Uh oh!

llvmbot commented Jul 16, 2024

Uh oh!

goldsteinn Jul 17, 2024

Choose a reason for hiding this comment

Uh oh!

dtcxzyw Jul 17, 2024

Choose a reason for hiding this comment

Uh oh!

dtcxzyw commented Jul 17, 2024

Uh oh!

Uh oh!

Uh oh!

Uh oh!

tgymnich commented Jul 17, 2024 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Uh oh!

dtcxzyw left a comment

Choose a reason for hiding this comment

Uh oh!

tgymnich commented Jul 17, 2024

Uh oh!

dtcxzyw commented Jul 17, 2024

Uh oh!

tgymnich commented Jul 17, 2024 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Uh oh!

sftlbcn commented Jul 17, 2024

Uh oh!

nikic left a comment

Choose a reason for hiding this comment

Uh oh!

tgymnich commented Jul 17, 2024

Uh oh!

Uh oh!

Uh oh!

tgymnich commented Jul 16, 2024 •

edited

Loading

tgymnich commented Jul 17, 2024 •

edited

Loading

tgymnich commented Jul 17, 2024 •

edited

Loading