[llvm] [SimplifyCFG] Treat umul + extract pattern as cheap single instruction (Approach 2) (PR #128021)

Gábor Spaits via llvm-commits llvm-commits at lists.llvm.org
Thu Feb 20 07:59:21 PST 2025


https://github.com/spaits updated https://github.com/llvm/llvm-project/pull/128021

>From 9b11238b239526b97a551250edbdabdd9ba94442 Mon Sep 17 00:00:00 2001
From: Gabor Spaits <gaborspaits1 at gmail.com>
Date: Thu, 20 Feb 2025 16:48:13 +0100
Subject: [PATCH 1/2] [SimplifyCFG] Treat umul + extract pattern as cheap
 single instruction

---
 llvm/lib/Transforms/Utils/SimplifyCFG.cpp     | 24 +++++---
 .../X86/unsigned-multiply-overflow-check.ll   | 34 +++---------
 .../SimplifyCFG/umul-extract-pattern.ll       | 55 +++++++++++++++++++
 3 files changed, 78 insertions(+), 35 deletions(-)
 create mode 100644 llvm/test/Transforms/SimplifyCFG/umul-extract-pattern.ll

diff --git a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
index 12dd49da279b9..157716dc7ead9 100644
--- a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
+++ b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
@@ -421,11 +421,11 @@ static InstructionCost computeSpeculationCost(const User *I,
 /// After this function returns, Cost is increased by the cost of
 /// V plus its non-dominating operands.  If that cost is greater than
 /// Budget, false is returned and Cost is undefined.
-static bool dominatesMergePoint(Value *V, BasicBlock *BB, Instruction *InsertPt,
-                                SmallPtrSetImpl<Instruction *> &AggressiveInsts,
-                                InstructionCost &Cost, InstructionCost Budget,
-                                const TargetTransformInfo &TTI,
-                                AssumptionCache *AC, unsigned Depth = 0) {
+static bool dominatesMergePoint(
+    Value *V, BasicBlock *BB, Instruction *InsertPt,
+    SmallPtrSetImpl<Instruction *> &AggressiveInsts, InstructionCost &Cost,
+    InstructionCost Budget, const TargetTransformInfo &TTI, AssumptionCache *AC,
+    SmallPtrSetImpl<Instruction *> &ZeroCostInstructions, unsigned Depth = 0) {
   // It is possible to hit a zero-cost cycle (phi/gep instructions for example),
   // so limit the recursion depth.
   // TODO: While this recursion limit does prevent pathological behavior, it
@@ -463,7 +463,12 @@ static bool dominatesMergePoint(Value *V, BasicBlock *BB, Instruction *InsertPt,
   if (!isSafeToSpeculativelyExecute(I, InsertPt, AC))
     return false;
 
-  Cost += computeSpeculationCost(I, TTI);
+  WithOverflowInst *OverflowInst;
+  if (match(I, m_ExtractValue<1>(m_OneUse(m_WithOverflowInst(OverflowInst))))) {
+    ZeroCostInstructions.insert(OverflowInst);
+    Cost += 1;
+  } else if (!ZeroCostInstructions.contains(I))
+    Cost += computeSpeculationCost(I, TTI);
 
   // Allow exactly one instruction to be speculated regardless of its cost
   // (as long as it is safe to do so).
@@ -480,7 +485,7 @@ static bool dominatesMergePoint(Value *V, BasicBlock *BB, Instruction *InsertPt,
   // not take us over the cost threshold.
   for (Use &Op : I->operands())
     if (!dominatesMergePoint(Op, BB, InsertPt, AggressiveInsts, Cost, Budget,
-                             TTI, AC, Depth + 1))
+                             TTI, AC, ZeroCostInstructions, Depth + 1))
       return false;
   // Okay, it's safe to do this!  Remember this instruction.
   AggressiveInsts.insert(I);
@@ -3810,6 +3815,7 @@ static bool foldTwoEntryPHINode(PHINode *PN, const TargetTransformInfo &TTI,
   // instructions.  While we are at it, keep track of the instructions
   // that need to be moved to the dominating block.
   SmallPtrSet<Instruction *, 4> AggressiveInsts;
+  SmallPtrSet<Instruction *, 2> ZeroCostInstructions;
   InstructionCost Cost = 0;
   InstructionCost Budget =
       TwoEntryPHINodeFoldingThreshold * TargetTransformInfo::TCC_Basic;
@@ -3827,9 +3833,9 @@ static bool foldTwoEntryPHINode(PHINode *PN, const TargetTransformInfo &TTI,
     }
 
     if (!dominatesMergePoint(PN->getIncomingValue(0), BB, DomBI,
-                             AggressiveInsts, Cost, Budget, TTI, AC) ||
+                             AggressiveInsts, Cost, Budget, TTI, AC, ZeroCostInstructions) ||
         !dominatesMergePoint(PN->getIncomingValue(1), BB, DomBI,
-                             AggressiveInsts, Cost, Budget, TTI, AC))
+                             AggressiveInsts, Cost, Budget, TTI, AC, ZeroCostInstructions))
       return Changed;
   }
 
diff --git a/llvm/test/Transforms/PhaseOrdering/X86/unsigned-multiply-overflow-check.ll b/llvm/test/Transforms/PhaseOrdering/X86/unsigned-multiply-overflow-check.ll
index 7bcb6ce17df0e..9858591dfc700 100644
--- a/llvm/test/Transforms/PhaseOrdering/X86/unsigned-multiply-overflow-check.ll
+++ b/llvm/test/Transforms/PhaseOrdering/X86/unsigned-multiply-overflow-check.ll
@@ -45,26 +45,17 @@ define i1 @will_not_overflow(i64 %arg, i64 %arg1) {
 ; INSTCOMBINESIMPLIFYCFGONLY-LABEL: @will_not_overflow(
 ; INSTCOMBINESIMPLIFYCFGONLY-NEXT:  bb:
 ; INSTCOMBINESIMPLIFYCFGONLY-NEXT:    [[T0:%.*]] = icmp eq i64 [[ARG:%.*]], 0
-; INSTCOMBINESIMPLIFYCFGONLY-NEXT:    br i1 [[T0]], label [[BB5:%.*]], label [[BB2:%.*]]
-; INSTCOMBINESIMPLIFYCFGONLY:       bb2:
 ; INSTCOMBINESIMPLIFYCFGONLY-NEXT:    [[MUL:%.*]] = call { i64, i1 } @llvm.umul.with.overflow.i64(i64 [[ARG]], i64 [[ARG1:%.*]])
 ; INSTCOMBINESIMPLIFYCFGONLY-NEXT:    [[MUL_OV:%.*]] = extractvalue { i64, i1 } [[MUL]], 1
-; INSTCOMBINESIMPLIFYCFGONLY-NEXT:    br label [[BB5]]
-; INSTCOMBINESIMPLIFYCFGONLY:       bb5:
-; INSTCOMBINESIMPLIFYCFGONLY-NEXT:    [[T6:%.*]] = phi i1 [ false, [[BB:%.*]] ], [ [[MUL_OV]], [[BB2]] ]
+; INSTCOMBINESIMPLIFYCFGONLY-NEXT:    [[T6:%.*]] = select i1 [[T0]], i1 false, i1 [[MUL_OV]]
 ; INSTCOMBINESIMPLIFYCFGONLY-NEXT:    ret i1 [[T6]]
 ;
 ; INSTCOMBINESIMPLIFYCFGINSTCOMBINE-LABEL: @will_not_overflow(
 ; INSTCOMBINESIMPLIFYCFGINSTCOMBINE-NEXT:  bb:
-; INSTCOMBINESIMPLIFYCFGINSTCOMBINE-NEXT:    [[T0:%.*]] = icmp eq i64 [[ARG:%.*]], 0
-; INSTCOMBINESIMPLIFYCFGINSTCOMBINE-NEXT:    br i1 [[T0]], label [[BB5:%.*]], label [[BB2:%.*]]
-; INSTCOMBINESIMPLIFYCFGINSTCOMBINE:       bb2:
-; INSTCOMBINESIMPLIFYCFGINSTCOMBINE-NEXT:    [[MUL:%.*]] = call { i64, i1 } @llvm.umul.with.overflow.i64(i64 [[ARG]], i64 [[ARG1:%.*]])
+; INSTCOMBINESIMPLIFYCFGINSTCOMBINE-NEXT:    [[ARG1:%.*]] = freeze i64 [[ARG2:%.*]]
+; INSTCOMBINESIMPLIFYCFGINSTCOMBINE-NEXT:    [[MUL:%.*]] = call { i64, i1 } @llvm.umul.with.overflow.i64(i64 [[ARG:%.*]], i64 [[ARG1]])
 ; INSTCOMBINESIMPLIFYCFGINSTCOMBINE-NEXT:    [[MUL_OV:%.*]] = extractvalue { i64, i1 } [[MUL]], 1
-; INSTCOMBINESIMPLIFYCFGINSTCOMBINE-NEXT:    br label [[BB5]]
-; INSTCOMBINESIMPLIFYCFGINSTCOMBINE:       bb5:
-; INSTCOMBINESIMPLIFYCFGINSTCOMBINE-NEXT:    [[T6:%.*]] = phi i1 [ false, [[BB:%.*]] ], [ [[MUL_OV]], [[BB2]] ]
-; INSTCOMBINESIMPLIFYCFGINSTCOMBINE-NEXT:    ret i1 [[T6]]
+; INSTCOMBINESIMPLIFYCFGINSTCOMBINE-NEXT:    ret i1 [[MUL_OV]]
 ;
 bb:
   %t0 = icmp eq i64 %arg, 0
@@ -112,28 +103,19 @@ define i1 @will_overflow(i64 %arg, i64 %arg1) {
 ; INSTCOMBINESIMPLIFYCFGONLY-LABEL: @will_overflow(
 ; INSTCOMBINESIMPLIFYCFGONLY-NEXT:  bb:
 ; INSTCOMBINESIMPLIFYCFGONLY-NEXT:    [[T0:%.*]] = icmp eq i64 [[ARG:%.*]], 0
-; INSTCOMBINESIMPLIFYCFGONLY-NEXT:    br i1 [[T0]], label [[BB5:%.*]], label [[BB2:%.*]]
-; INSTCOMBINESIMPLIFYCFGONLY:       bb2:
 ; INSTCOMBINESIMPLIFYCFGONLY-NEXT:    [[MUL:%.*]] = call { i64, i1 } @llvm.umul.with.overflow.i64(i64 [[ARG]], i64 [[ARG1:%.*]])
 ; INSTCOMBINESIMPLIFYCFGONLY-NEXT:    [[MUL_OV:%.*]] = extractvalue { i64, i1 } [[MUL]], 1
 ; INSTCOMBINESIMPLIFYCFGONLY-NEXT:    [[PHI_BO:%.*]] = xor i1 [[MUL_OV]], true
-; INSTCOMBINESIMPLIFYCFGONLY-NEXT:    br label [[BB5]]
-; INSTCOMBINESIMPLIFYCFGONLY:       bb5:
-; INSTCOMBINESIMPLIFYCFGONLY-NEXT:    [[T6:%.*]] = phi i1 [ true, [[BB:%.*]] ], [ [[PHI_BO]], [[BB2]] ]
+; INSTCOMBINESIMPLIFYCFGONLY-NEXT:    [[T6:%.*]] = select i1 [[T0]], i1 true, i1 [[PHI_BO]]
 ; INSTCOMBINESIMPLIFYCFGONLY-NEXT:    ret i1 [[T6]]
 ;
 ; INSTCOMBINESIMPLIFYCFGINSTCOMBINE-LABEL: @will_overflow(
 ; INSTCOMBINESIMPLIFYCFGINSTCOMBINE-NEXT:  bb:
-; INSTCOMBINESIMPLIFYCFGINSTCOMBINE-NEXT:    [[T0:%.*]] = icmp eq i64 [[ARG:%.*]], 0
-; INSTCOMBINESIMPLIFYCFGINSTCOMBINE-NEXT:    br i1 [[T0]], label [[BB5:%.*]], label [[BB2:%.*]]
-; INSTCOMBINESIMPLIFYCFGINSTCOMBINE:       bb2:
-; INSTCOMBINESIMPLIFYCFGINSTCOMBINE-NEXT:    [[MUL:%.*]] = call { i64, i1 } @llvm.umul.with.overflow.i64(i64 [[ARG]], i64 [[ARG1:%.*]])
+; INSTCOMBINESIMPLIFYCFGINSTCOMBINE-NEXT:    [[ARG1:%.*]] = freeze i64 [[ARG2:%.*]]
+; INSTCOMBINESIMPLIFYCFGINSTCOMBINE-NEXT:    [[MUL:%.*]] = call { i64, i1 } @llvm.umul.with.overflow.i64(i64 [[ARG:%.*]], i64 [[ARG1]])
 ; INSTCOMBINESIMPLIFYCFGINSTCOMBINE-NEXT:    [[MUL_OV:%.*]] = extractvalue { i64, i1 } [[MUL]], 1
 ; INSTCOMBINESIMPLIFYCFGINSTCOMBINE-NEXT:    [[PHI_BO:%.*]] = xor i1 [[MUL_OV]], true
-; INSTCOMBINESIMPLIFYCFGINSTCOMBINE-NEXT:    br label [[BB5]]
-; INSTCOMBINESIMPLIFYCFGINSTCOMBINE:       bb5:
-; INSTCOMBINESIMPLIFYCFGINSTCOMBINE-NEXT:    [[T6:%.*]] = phi i1 [ true, [[BB:%.*]] ], [ [[PHI_BO]], [[BB2]] ]
-; INSTCOMBINESIMPLIFYCFGINSTCOMBINE-NEXT:    ret i1 [[T6]]
+; INSTCOMBINESIMPLIFYCFGINSTCOMBINE-NEXT:    ret i1 [[PHI_BO]]
 ;
 bb:
   %t0 = icmp eq i64 %arg, 0
diff --git a/llvm/test/Transforms/SimplifyCFG/umul-extract-pattern.ll b/llvm/test/Transforms/SimplifyCFG/umul-extract-pattern.ll
new file mode 100644
index 0000000000000..8a5e034998a25
--- /dev/null
+++ b/llvm/test/Transforms/SimplifyCFG/umul-extract-pattern.ll
@@ -0,0 +1,55 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt -passes=simplifycfg -simplifycfg-require-and-preserve-domtree=1 -S < %s | FileCheck %s
+target triple = "riscv64-unknown-unknown-elf"
+
+define i16 @func2(i64 %x, i64 %y) {
+; CHECK-LABEL: @func2(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[CMP_NOT:%.*]] = icmp eq i64 [[Y:%.*]], 0
+; CHECK-NEXT:    [[MUL:%.*]] = tail call { i64, i1 } @llvm.umul.with.overflow.i64(i64 [[Y]], i64 [[X:%.*]])
+; CHECK-NEXT:    [[MUL_OV:%.*]] = extractvalue { i64, i1 } [[MUL]], 1
+; CHECK-NEXT:    [[SPEC_SELECT:%.*]] = select i1 [[CMP_NOT]], i1 false, i1 [[MUL_OV]]
+; CHECK-NEXT:    [[CONV:%.*]] = zext i1 [[SPEC_SELECT]] to i16
+; CHECK-NEXT:    ret i16 [[CONV]]
+;
+entry:
+  %cmp.not = icmp eq i64 %y, 0
+  br i1 %cmp.not, label %land.end, label %land.rhs
+
+land.rhs:                                         ; preds = %entry
+  %mul = tail call { i64, i1 } @llvm.umul.with.overflow.i64(i64 %y, i64 %x)
+  %mul.ov = extractvalue { i64, i1 } %mul, 1
+  br label %land.end
+
+land.end:                                         ; preds = %land.rhs, %entry
+  %0 = phi i1 [ false, %entry ], [ %mul.ov, %land.rhs ]
+  %conv = zext i1 %0 to i16
+  ret i16 %conv
+}
+
+define i16 @noHoist(i64 %x, i64 %y) {
+; CHECK-LABEL: @noHoist(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[CMP_NOT:%.*]] = icmp eq i64 [[Y:%.*]], 0
+; CHECK-NEXT:    [[ADD2:%.*]] = add nsw i64 [[Y]], [[X:%.*]]
+; CHECK-NEXT:    [[MUL:%.*]] = tail call { i64, i1 } @llvm.umul.with.overflow.i64(i64 [[ADD2]], i64 [[X]])
+; CHECK-NEXT:    [[MUL_OV:%.*]] = extractvalue { i64, i1 } [[MUL]], 1
+; CHECK-NEXT:    [[SPEC_SELECT:%.*]] = select i1 [[CMP_NOT]], i1 false, i1 [[MUL_OV]]
+; CHECK-NEXT:    [[CONV:%.*]] = zext i1 [[SPEC_SELECT]] to i16
+; CHECK-NEXT:    ret i16 [[CONV]]
+;
+entry:
+  %cmp.not = icmp eq i64 %y, 0
+  br i1 %cmp.not, label %land.end, label %land.rhs
+
+land.rhs:                                   ; preds = %entry
+  %add = add nsw i64 %y, %x
+  %mul = tail call { i64, i1 } @llvm.umul.with.overflow.i64(i64 %add, i64 %x)
+  %mul.ov = extractvalue { i64, i1 } %mul, 1
+  br label %land.end
+
+land.end:                                         ; preds = %land.rhs, %entry
+  %0 = phi i1 [ false, %entry ], [ %mul.ov, %land.rhs ]
+  %conv = zext i1 %0 to i16
+  ret i16 %conv
+}

>From ea09c0b64c399f8ce1db43447155fc064805ba4f Mon Sep 17 00:00:00 2001
From: Gabor Spaits <gaborspaits1 at gmail.com>
Date: Thu, 20 Feb 2025 16:59:08 +0100
Subject: [PATCH 2/2] Fix formatting

---
 llvm/lib/Transforms/Utils/SimplifyCFG.cpp | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
index 157716dc7ead9..b9bc32084ab2e 100644
--- a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
+++ b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
@@ -3833,9 +3833,11 @@ static bool foldTwoEntryPHINode(PHINode *PN, const TargetTransformInfo &TTI,
     }
 
     if (!dominatesMergePoint(PN->getIncomingValue(0), BB, DomBI,
-                             AggressiveInsts, Cost, Budget, TTI, AC, ZeroCostInstructions) ||
+                             AggressiveInsts, Cost, Budget, TTI, AC,
+                             ZeroCostInstructions) ||
         !dominatesMergePoint(PN->getIncomingValue(1), BB, DomBI,
-                             AggressiveInsts, Cost, Budget, TTI, AC, ZeroCostInstructions))
+                             AggressiveInsts, Cost, Budget, TTI, AC,
+                             ZeroCostInstructions))
       return Changed;
   }
 



More information about the llvm-commits mailing list