[llvm] r244887 - [InstCombinePHI] Partial simplification of identity operations.
Charlie Turner via llvm-commits
llvm-commits at lists.llvm.org
Thu Aug 13 05:38:59 PDT 2015
Author: chatur01
Date: Thu Aug 13 07:38:58 2015
New Revision: 244887
URL: http://llvm.org/viewvc/llvm-project?rev=244887&view=rev
Log:
[InstCombinePHI] Partial simplification of identity operations.
Consider this code:
BB:
%i = phi i32 [ 0, %if.then ], [ %c, %if.else ]
%add = add nsw i32 %i, %b
...
In this common case the add can be moved to the %if.else basic block, because
adding zero is an identity operation. If we go though %if.then branch it's
always a win, because add is not executed; if not, the number of instructions
stays the same.
This pattern applies also to other instructions like sub, shl, shr, ashr | 0,
mul, sdiv, div | 1.
Patch by Jakub Kuderski!
Modified:
llvm/trunk/lib/Transforms/InstCombine/InstCombinePHI.cpp
llvm/trunk/test/Transforms/InstCombine/phi.ll
Modified: llvm/trunk/lib/Transforms/InstCombine/InstCombinePHI.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/InstCombine/InstCombinePHI.cpp?rev=244887&r1=244886&r2=244887&view=diff
==============================================================================
--- llvm/trunk/lib/Transforms/InstCombine/InstCombinePHI.cpp (original)
+++ llvm/trunk/lib/Transforms/InstCombine/InstCombinePHI.cpp Thu Aug 13 07:38:58 2015
@@ -781,6 +781,106 @@ Instruction *InstCombiner::SliceUpIllega
return ReplaceInstUsesWith(FirstPhi, Undef);
}
+// If a PHI node has two edges and the PHI node is used in instructions like
+// add, sub, mul, div, shifts; if one of incoming values is a constant
+// that makes the instruction and identity operation, we can hoist this
+// instruction into one of the basic blocks.
+// Because of such a transformation, the identity operation won't be
+// executed, since it doesn't contribute to the result.
+//
+static Instruction *PartiallySimplifyIdentityOps(PHINode &PN, const Constant &C,
+ Value &IncomingVal,
+ Instruction &PNUser,
+ InstCombiner &IC) {
+ if (!PNUser.isBinaryOp())
+ return nullptr;
+ if (PN.getParent() != PNUser.getParent())
+ return nullptr;
+
+ // C IncomingVal
+ // \ /
+ // \ 0 1 / -- (IncomingValIdx)
+ // \ /
+ // PN = phi [C , ...] [ IncomingVal, BB ]
+ // ...
+ // 0 1 -- (OpOperandIdx)
+ // PNUser = op PN, x
+ const unsigned IncomingValIdx =
+ (&IncomingVal == PN.getIncomingValue(0)) ? 0 : 1;
+ const unsigned OpOperandIdx = (&PN == PNUser.getOperand(0)) ? 1 : 0;
+
+ // Exit if not an identity operation.
+ // For everything except add Add and Mul constant must be on the RHS.
+ switch (PNUser.getOpcode()) {
+ default:
+ return nullptr;
+ case Instruction::Add:
+ if (!C.isZeroValue())
+ return nullptr;
+ break;
+
+ case Instruction::Sub:
+ case Instruction::Shl:
+ case Instruction::LShr:
+ case Instruction::AShr:
+ if (!C.isZeroValue() || OpOperandIdx == 1)
+ return nullptr;
+ break;
+
+ case Instruction::Mul:
+ if (!C.isOneValue())
+ return nullptr;
+ break;
+
+ case Instruction::UDiv:
+ case Instruction::SDiv:
+ if (!C.isOneValue() || OpOperandIdx == 1)
+ return nullptr;
+ break;
+ }
+
+ BasicBlock *BB = PN.getIncomingBlock(IncomingValIdx);
+ auto *Terminator = BB->getTerminator();
+
+ if (const auto *Incoming = dyn_cast<Instruction>(&IncomingVal))
+ if (!IC.getDominatorTree()->dominates(Incoming, Terminator))
+ return nullptr;
+
+ // Operand must be available in newly generated instruction and
+ // as an incoming value of the PHI node.
+ if (const auto *Operand =
+ dyn_cast<Instruction>(PNUser.getOperand(OpOperandIdx)))
+ if (!IC.getDominatorTree()->dominates(Operand, Terminator) ||
+ !IC.getDominatorTree()->dominates(Operand, &PN))
+ return nullptr;
+
+ // Ensure that the non-constant value in the PHI node doesn't come
+ // from the same BasicBlock as the PHI node. This prevents errors
+ // that could appear with loops (loop backedge could have this
+ // problem).
+ if (PN.getIncomingBlock(IncomingValIdx) == PN.getParent())
+ return nullptr;
+
+ Value *LHS = &IncomingVal, *RHS = PNUser.getOperand(OpOperandIdx);
+ if (OpOperandIdx == 0)
+ std::swap(LHS, RHS);
+
+ // Add new instruction to one of the edges.
+ IRBuilder<> Builder(Terminator);
+ auto *NewInst =
+ Builder.CreateBinOp(cast<BinaryOperator>(PNUser).getOpcode(), LHS, RHS);
+ cast<BinaryOperator>(NewInst)->copyIRFlags(&PNUser);
+
+ // The new incoming values are:
+ // - result of the newly emmited instruction
+ // - operand of the instruction
+ PN.setIncomingValue(IncomingValIdx, NewInst);
+ PN.setIncomingValue(IncomingValIdx == 0 ? 1 : 0,
+ PNUser.getOperand(OpOperandIdx));
+ IC.ReplaceInstUsesWith(PNUser, &PN);
+ return &PN;
+}
+
// PHINode simplification
//
Instruction *InstCombiner::visitPHINode(PHINode &PN) {
@@ -822,6 +922,21 @@ Instruction *InstCombiner::visitPHINode(
PHIUser->user_back() == &PN) {
return ReplaceInstUsesWith(PN, UndefValue::get(PN.getType()));
}
+
+ // If this phi has one use, exactly 2 edges and one is a constant, we
+ // may be able to apply the PartiallySimplifyIdentityOps optimization.
+ if (PN.getNumIncomingValues() == 2) {
+ const Constant *C = dyn_cast<Constant>(PN.getIncomingValue(0));
+ Value *Val = PN.getIncomingValue(1);
+ if (!C) {
+ C = dyn_cast<Constant>(PN.getIncomingValue(1));
+ Val = PN.getIncomingValue(0);
+ }
+ if (C && !isa<Constant>(Val))
+ if (auto *I =
+ PartiallySimplifyIdentityOps(PN, *C, *Val, *PHIUser, *this))
+ return I;
+ }
}
// We sometimes end up with phi cycles that non-obviously end up being the
Modified: llvm/trunk/test/Transforms/InstCombine/phi.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/InstCombine/phi.ll?rev=244887&r1=244886&r2=244887&view=diff
==============================================================================
--- llvm/trunk/test/Transforms/InstCombine/phi.ll (original)
+++ llvm/trunk/test/Transforms/InstCombine/phi.ll Thu Aug 13 07:38:58 2015
@@ -247,8 +247,9 @@ end:
ret i64 %tmp2
; CHECK-LABEL: @test12(
; CHECK-NOT: zext
+; CHECK: %[[X:.*]] = add i64 %{{.*}}, %Val
; CHECK: end:
-; CHECK-NEXT: phi i64 [ 0, %entry ], [ %Val, %two ]
+; CHECK-NEXT: phi i64 [ %tmp41, %entry ], [ %[[X]], %two ]
; CHECK-NOT: phi
; CHECK: ret i64
}
@@ -630,3 +631,124 @@ done:
%y = phi i32 [ undef, %entry ]
ret i32 %y
}
+
+; CHECK-LABEL: @test28(
+; CHECK add nsw i32 %x, %dfd
+; CHECK: if.end:
+; CHECK-NEXT: phi i32 [ %{{.*}}, %if.else ], [ %x, %entry ]
+; CHECK-NEXT: ret
+define i32 @test28(i1* nocapture readonly %cond, i32* nocapture readonly %a, i32 %x, i32 %v) {
+entry:
+ %0 = load volatile i1, i1* %cond, align 1
+ br i1 %0, label %if.else, label %if.end
+
+if.else:
+ %1 = load volatile i32, i32* %a, align 4
+ br label %if.end
+
+if.end:
+ %v.addr.0 = phi i32 [ %1, %if.else ], [ 0, %entry ]
+ %div = add nsw i32 %x, %v.addr.0
+ ret i32 %div
+}
+
+; CHECK-LABEL: @test29(
+; CHECK: ashr i32 %x, %1
+; CHECK: if.end:
+; CHECK-NEXT: phi i32 [ %{{.*}}, %if.else ], [ %x, %entry ]
+; CHECK-NEXT: ret
+define i32 @test29(i1* nocapture readonly %cond, i32* nocapture readonly %a, i32 %x, i32 %v) {
+entry:
+ %0 = load volatile i1, i1* %cond, align 1
+ br i1 %0, label %if.else, label %if.end
+
+if.else:
+ %1 = load volatile i32, i32* %a, align 4
+ br label %if.end
+
+if.end:
+ %v.addr.0 = phi i32 [ %1, %if.else ], [ 0, %entry ]
+ %div = ashr i32 %x, %v.addr.0
+ ret i32 %div
+}
+
+; CHECK-LABEL: @test30(
+; CHECK: sdiv i32 %x, %1
+; CHECK: if.end:
+; CHECK-NEXT: phi i32 [ %{{.*}}, %if.else ], [ %x, %entry ]
+; CHECK-NEXT: ret
+define i32 @test30(i1* nocapture readonly %cond, i32* nocapture readonly %a, i32 %x, i32 %v) {
+entry:
+ %0 = load volatile i1, i1* %cond, align 1
+ br i1 %0, label %if.else, label %if.end
+
+if.else:
+ %1 = load volatile i32, i32* %a, align 4
+ br label %if.end
+
+if.end:
+ %v.addr.0 = phi i32 [ %1, %if.else ], [ 1, %entry ]
+ %div = sdiv i32 %x, %v.addr.0
+ ret i32 %div
+}
+
+; CHECK-LABEL: @test31(
+; CHECK: mul i32
+; CHECK: if.end:
+; CHECK-NEXT: phi i32 [ %{{.*}}, %if.else ], [ %x, %entry ]
+; CHECK-NEXT: ret
+define i32 @test31(i1* nocapture readonly %cond, i32* nocapture readonly %a, i32 %x, i32 %v) {
+entry:
+ %0 = load volatile i1, i1* %cond, align 1
+ br i1 %0, label %if.else, label %if.end
+
+if.else:
+ %1 = load volatile i32, i32* %a, align 4
+ br label %if.end
+
+if.end:
+ %v.addr.0 = phi i32 [ %1, %if.else ], [ 1, %entry ]
+ %div = mul i32 %x, %v.addr.0
+ ret i32 %div
+}
+
+; CHECK-LABEL: @test32(
+; CHECK: sdiv i32 %x, %1
+; CHECK: if.end:
+; CHECK-NEXT: phi i32 [ %x, %entry ], [ %{{.*}}, %if.else ]
+; CHECK-NEXT: ret
+define i32 @test32(i1* nocapture readonly %cond, i32* nocapture readonly %a, i32 %x, i32 %v) {
+entry:
+ %0 = load volatile i1, i1* %cond, align 1
+ br i1 %0, label %if.else, label %if.end
+
+if.else:
+ %1 = load volatile i32, i32* %a, align 4
+ br label %if.end
+
+if.end:
+ %v.addr.0 = phi i32 [ 1, %entry ], [ %1, %if.else ]
+ %div = sdiv i32 %x, %v.addr.0
+ ret i32 %div
+}
+
+; Constant (0) is on the LHS of shift - should not aplpy the transformation
+; CHECK-LABEL: @test33(
+; CHECK: if.end:
+; CHECK-NEXT: %v.addr.0 = phi i32 [ 0, %entry ], [ %1, %if.else ]
+; CHECK-NEXT: shl
+; CHECK-NEXT: ret
+define i32 @test33(i1* nocapture readonly %cond, i32* nocapture readonly %a, i32 %x, i32 %v) {
+entry:
+ %0 = load volatile i1, i1* %cond, align 1
+ br i1 %0, label %if.else, label %if.end
+
+if.else:
+ %1 = load volatile i32, i32* %a, align 4
+ br label %if.end
+
+if.end:
+ %v.addr.0 = phi i32 [ 0, %entry ], [ %1, %if.else ]
+ %div = shl i32 %v.addr.0, %x
+ ret i32 %div
+}
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