[llvm-commits] [llvm] r164173 - in /llvm/trunk: include/llvm/Transforms/Utils/IntegerDivision.h lib/Transforms/Utils/CMakeLists.txt lib/Transforms/Utils/IntegerDivision.cpp
Michael Ilseman
milseman at apple.com
Wed Sep 19 08:39:03 PDT 2012
Thanks for the review! I will address these later today.
On Sep 18, 2012, at 7:17 PM, Chandler Carruth <chandlerc at google.com> wrote:
> I'm sad this didn't get any pre-commit review on the public mailing lists...
>
> On Tue, Sep 18, 2012 at 3:02 PM, Michael Ilseman <milseman at apple.com> wrote:
> Author: milseman
> Date: Tue Sep 18 17:02:40 2012
> New Revision: 164173
>
> URL: http://llvm.org/viewvc/llvm-project?rev=164173&view=rev
> Log:
> New utility for expanding integer division for targets that don't support it.
>
> Implementation derived from compiler-rt's implementation of signed and unsigned integer division.
>
> There are neither users nor tests of this code in the tree. That is a really bad state. This will bitrot immediately. Please write thorough tests for this if you want to keep it in the tree as a utility for out-of-tree projects. Other developers have no other way to exercise it.
>
> Also:
>
> +namespace llvm {
> +
> + bool expandDivision(BinaryOperator* Div);
>
> Zero comments? No Doxygen? This hardly seems appropriate. I realize the external behavior is simple, but that doesn't mean it can go without documentation.
>
>
> +
> +} // End llvm namespace
> +
> +#endif
>
> Modified: llvm/trunk/lib/Transforms/Utils/CMakeLists.txt
> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/Utils/CMakeLists.txt?rev=164173&r1=164172&r2=164173&view=diff
> ==============================================================================
> --- llvm/trunk/lib/Transforms/Utils/CMakeLists.txt (original)
> +++ llvm/trunk/lib/Transforms/Utils/CMakeLists.txt Tue Sep 18 17:02:40 2012
> @@ -11,6 +11,7 @@
> DemoteRegToStack.cpp
> InlineFunction.cpp
> InstructionNamer.cpp
> + IntegerDivision.cpp
> LCSSA.cpp
> Local.cpp
> LoopSimplify.cpp
>
> Added: llvm/trunk/lib/Transforms/Utils/IntegerDivision.cpp
> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/Utils/IntegerDivision.cpp?rev=164173&view=auto
> ==============================================================================
> --- llvm/trunk/lib/Transforms/Utils/IntegerDivision.cpp (added)
> +++ llvm/trunk/lib/Transforms/Utils/IntegerDivision.cpp Tue Sep 18 17:02:40 2012
> @@ -0,0 +1,306 @@
> +//===-- IntegerDivision.cpp - Expand integer division ---------------------===//
> +//
> +// The LLVM Compiler Infrastructure
> +//
> +// This file is distributed under the University of Illinois Open Source
> +// License. See LICENSE.TXT for details.
> +//
> +//===----------------------------------------------------------------------===//
> +//
> +// This file contains an implementation of 32bit scalar integer division for
> +// targets that don't have native support. It's largely derived from
> +// compiler-rt's implementation of __udivsi3, but hand-tuned to reduce the
> +// amount of control flow
> +//
> +//===----------------------------------------------------------------------===//
> +
> +#define DEBUG_TYPE "integer-division"
> +#include "llvm/Function.h"
> +#include "llvm/Instructions.h"
> +#include "llvm/Intrinsics.h"
> +#include "llvm/IRBuilder.h"
> +#include "llvm/Transforms/Utils/IntegerDivision.h"
> +
> +using namespace llvm;
> +
> +// Generate code to divide two signed integers. Returns the quotient, rounded
> +// towards 0. Builder's insert point should be pointing at the sdiv
> +// instruction. This will generate a udiv in the process, and Builder's insert
> +// point will be pointing at the udiv (if present, i.e. not folded), ready to be
> +// expanded if the user wishes.
> +static Value* GenerateSignedDivisionCode(Value* Dividend, Value* Divisor,
> + IRBuilder<>& Builder) {
> + // Implementation taken from compiler-rt's __divsi3
> +
> + ConstantInt* ThirtyOne = Builder.getInt32(31);
>
> The "*" are consistently in the wrong place. Also this doesn't follow the style guide's naming conventions. Also it should have a doxygen comment.
>
> +
> + // ; %tmp = ashr i32 %dividend, 31
> + // ; %tmp1 = ashr i32 %divisor, 31
> + // ; %tmp2 = xor i32 %tmp, %dividend
> + // ; %u_dvnd = sub nsw i32 %tmp2, %tmp
> + // ; %tmp3 = xor i32 %tmp1, %divisor
> + // ; %u_dvsr = sub nsw i32 %tmp3, %tmp1
> + // ; %q_sgn = xor i32 %tmp1, %tmp
> + // ; %q_mag = udiv i32 %u_dvnd, %u_dvsr
> + // ; %tmp4 = xor i32 %q_mag, %q_sgn
> + // ; %q = sub i32 %tmp4, %q_sgn
> + Value* Tmp = Builder.CreateAShr(Dividend, ThirtyOne);
> + Value* Tmp1 = Builder.CreateAShr(Divisor, ThirtyOne);
> + Value* Tmp2 = Builder.CreateXor(Tmp, Dividend);
> + Value* U_Dvnd = Builder.CreateSub(Tmp2, Tmp);
>
> You've given these variables terribly hard to read names when the length of name costs nothing. Could use use something more readable?
>
> Also, You've not given any names to the actual instructions being built, which will make reading the IR produced by this much harder.
>
> + Value* Tmp3 = Builder.CreateXor(Tmp1, Divisor);
> + Value* U_Dvsr = Builder.CreateSub(Tmp3, Tmp1);
> + Value* Q_Sgn = Builder.CreateXor(Tmp1, Tmp);
> + Value* Q_Mag = Builder.CreateUDiv(U_Dvnd, U_Dvsr);
> + Value* Tmp4 = Builder.CreateXor(Q_Mag, Q_Sgn);
> + Value* Q = Builder.CreateSub(Tmp4, Q_Sgn);
> +
> + if (Instruction* UDiv = dyn_cast<Instruction>(Q_Mag))
> + Builder.SetInsertPoint(UDiv);
> +
> + return Q;
> +}
> +
> +// Generates code to divide two unsigned scalar 32-bit integers. Returns the
> +// quotient, rounded towards 0. Builder's insert point should be pointing at the
> +// udiv instruction.
> +static Value* GenerateUnsignedDivisionCode(Value* Dividend, Value* Divisor,
> + IRBuilder<>& Builder) {
> + // The basic algorithm can be found in the compiler-rt project's
> + // implementation of __udivsi3.c. Here, we do a lower-level IR based approach
> + // that's been hand-tuned to lessen the amount of control flow involved.
> +
> + // Some helper values
> + IntegerType* I32Ty = Builder.getInt32Ty();
> +
> + ConstantInt* Zero = Builder.getInt32(0);
> + ConstantInt* One = Builder.getInt32(1);
> + ConstantInt* ThirtyOne = Builder.getInt32(31);
> + ConstantInt* NegOne = ConstantInt::getSigned(I32Ty, -1);
> + ConstantInt* True = Builder.getTrue();
> +
> + BasicBlock* IBB = Builder.GetInsertBlock();
> + Function* F = IBB->getParent();
> + Function* CTLZi32 = Intrinsic::getDeclaration(F->getParent(), Intrinsic::ctlz,
> + I32Ty);
> +
> + // Our CFG is going to look like:
> + // +---------------------+
> + // | special-cases |
> + // | ... |
> + // +---------------------+
> + // | |
> + // | +----------+
> + // | | bb1 |
> + // | | ... |
> + // | +----------+
> + // | | |
> + // | | +------------+
> + // | | | preheader |
> + // | | | ... |
> + // | | +------------+
> + // | | |
> + // | | | +---+
> + // | | | | |
> + // | | +------------+ |
> + // | | | do-while | |
> + // | | | ... | |
> + // | | +------------+ |
> + // | | | | |
> + // | +-----------+ +---+
> + // | | loop-exit |
> + // | | ... |
> + // | +-----------+
> + // | |
> + // +-------+
> + // | ... |
> + // | end |
> + // +-------+
> + BasicBlock* SpecialCases = Builder.GetInsertBlock();
> + SpecialCases->setName(Twine(SpecialCases->getName(), "_udiv-special-cases"));
> + BasicBlock* End = SpecialCases->splitBasicBlock(Builder.GetInsertPoint(),
> + "udiv-end");
> + BasicBlock* LoopExit = BasicBlock::Create(Builder.getContext(),
> + "udiv-loop-exit", F, End);
> + BasicBlock* DoWhile = BasicBlock::Create(Builder.getContext(),
> + "udiv-do-while", F, End);
> + BasicBlock* Preheader = BasicBlock::Create(Builder.getContext(),
> + "udiv-preheader", F, End);
> + BasicBlock* BB1 = BasicBlock::Create(Builder.getContext(),
> + "udiv-bb1", F, End);
> +
> + // We'll be overwriting the terminator to insert our extra blocks
> + SpecialCases->getTerminator()->eraseFromParent();
> +
> + // First off, check for special cases: dividend or divisor is zero, divisor
> + // is greater than dividend, and divisor is 1.
> + // ; special-cases:
> + // ; %ret0_1 = icmp eq i32 %divisor, 0
> + // ; %ret0_2 = icmp eq i32 %dividend, 0
> + // ; %ret0_3 = or i1 %ret0_1, %ret0_2
> + // ; %tmp0 = tail call i32 @llvm.ctlz.i32(i32 %divisor, i1 true)
> + // ; %tmp1 = tail call i32 @llvm.ctlz.i32(i32 %dividend, i1 true)
> + // ; %sr = sub nsw i32 %tmp0, %tmp1
> + // ; %ret0_4 = icmp ugt i32 %sr, 31
> + // ; %ret0 = or i1 %ret0_3, %ret0_4
> + // ; %retDividend = icmp eq i32 %sr, 31
> + // ; %retVal = select i1 %ret0, i32 0, i32 %dividend
> + // ; %earlyRet = or i1 %ret0, %retDividend
> + // ; br i1 %earlyRet, label %end, label %bb1
> + Builder.SetInsertPoint(SpecialCases);
> + Value* Ret0_1 = Builder.CreateICmpEQ(Divisor, Zero);
> + Value* Ret0_2 = Builder.CreateICmpEQ(Dividend, Zero);
> + Value* Ret0_3 = Builder.CreateOr(Ret0_1, Ret0_2);
> + Value* Tmp0 = Builder.CreateCall2(CTLZi32, Divisor, True);
> + Value* Tmp1 = Builder.CreateCall2(CTLZi32, Dividend, True);
> + Value* SR = Builder.CreateSub(Tmp0, Tmp1);
> + Value* Ret0_4 = Builder.CreateICmpUGT(SR, ThirtyOne);
> + Value* Ret0 = Builder.CreateOr(Ret0_3, Ret0_4);
> + Value* RetDividend = Builder.CreateICmpEQ(SR, ThirtyOne);
> + Value* RetVal = Builder.CreateSelect(Ret0, Zero, Dividend);
> + Value* EarlyRet = Builder.CreateOr(Ret0, RetDividend);
> + Builder.CreateCondBr(EarlyRet, End, BB1);
> +
> + // ; bb1: ; preds = %special-cases
> + // ; %sr_1 = add i32 %sr, 1
> + // ; %tmp2 = sub i32 31, %sr
> + // ; %q = shl i32 %dividend, %tmp2
> + // ; %skipLoop = icmp eq i32 %sr_1, 0
> + // ; br i1 %skipLoop, label %loop-exit, label %preheader
> + Builder.SetInsertPoint(BB1);
> + Value* SR_1 = Builder.CreateAdd(SR, One);
> + Value* Tmp2 = Builder.CreateSub(ThirtyOne, SR);
> + Value* Q = Builder.CreateShl(Dividend, Tmp2);
> + Value* SkipLoop = Builder.CreateICmpEQ(SR_1, Zero);
> + Builder.CreateCondBr(SkipLoop, LoopExit, Preheader);
> +
> + // ; preheader: ; preds = %bb1
> + // ; %tmp3 = lshr i32 %dividend, %sr_1
> + // ; %tmp4 = add i32 %divisor, -1
> + // ; br label %do-while
> + Builder.SetInsertPoint(Preheader);
> + Value* Tmp3 = Builder.CreateLShr(Dividend, SR_1);
> + Value* Tmp4 = Builder.CreateAdd(Divisor, NegOne);
> + Builder.CreateBr(DoWhile);
> +
> + // ; do-while: ; preds = %do-while, %preheader
> + // ; %carry_1 = phi i32 [ 0, %preheader ], [ %carry, %do-while ]
> + // ; %sr_3 = phi i32 [ %sr_1, %preheader ], [ %sr_2, %do-while ]
> + // ; %r_1 = phi i32 [ %tmp3, %preheader ], [ %r, %do-while ]
> + // ; %q_2 = phi i32 [ %q, %preheader ], [ %q_1, %do-while ]
> + // ; %tmp5 = shl i32 %r_1, 1
> + // ; %tmp6 = lshr i32 %q_2, 31
> + // ; %tmp7 = or i32 %tmp5, %tmp6
> + // ; %tmp8 = shl i32 %q_2, 1
> + // ; %q_1 = or i32 %carry_1, %tmp8
> + // ; %tmp9 = sub i32 %tmp4, %tmp7
> + // ; %tmp10 = ashr i32 %tmp9, 31
> + // ; %carry = and i32 %tmp10, 1
> + // ; %tmp11 = and i32 %tmp10, %divisor
> + // ; %r = sub i32 %tmp7, %tmp11
> + // ; %sr_2 = add i32 %sr_3, -1
> + // ; %tmp12 = icmp eq i32 %sr_2, 0
> + // ; br i1 %tmp12, label %loop-exit, label %do-while
> + Builder.SetInsertPoint(DoWhile);
> + PHINode* Carry_1 = Builder.CreatePHI(I32Ty, 2);
> + PHINode* SR_3 = Builder.CreatePHI(I32Ty, 2);
> + PHINode* R_1 = Builder.CreatePHI(I32Ty, 2);
> + PHINode* Q_2 = Builder.CreatePHI(I32Ty, 2);
> + Value* Tmp5 = Builder.CreateShl(R_1, One);
> + Value* Tmp6 = Builder.CreateLShr(Q_2, ThirtyOne);
> + Value* Tmp7 = Builder.CreateOr(Tmp5, Tmp6);
> + Value* Tmp8 = Builder.CreateShl(Q_2, One);
> + Value* Q_1 = Builder.CreateOr(Carry_1, Tmp8);
> + Value* Tmp9 = Builder.CreateSub(Tmp4, Tmp7);
> + Value* Tmp10 = Builder.CreateAShr(Tmp9, 31);
> + Value* Carry = Builder.CreateAnd(Tmp10, One);
> + Value* Tmp11 = Builder.CreateAnd(Tmp10, Divisor);
> + Value* R = Builder.CreateSub(Tmp7, Tmp11);
> + Value* SR_2 = Builder.CreateAdd(SR_3, NegOne);
> + Value* Tmp12 = Builder.CreateICmpEQ(SR_2, Zero);
> + Builder.CreateCondBr(Tmp12, LoopExit, DoWhile);
> +
> + // ; loop-exit: ; preds = %do-while, %bb1
> + // ; %carry_2 = phi i32 [ 0, %bb1 ], [ %carry, %do-while ]
> + // ; %q_3 = phi i32 [ %q, %bb1 ], [ %q_1, %do-while ]
> + // ; %tmp13 = shl i32 %q_3, 1
> + // ; %q_4 = or i32 %carry_2, %tmp13
> + // ; br label %end
> + Builder.SetInsertPoint(LoopExit);
> + PHINode* Carry_2 = Builder.CreatePHI(I32Ty, 2);
> + PHINode* Q_3 = Builder.CreatePHI(I32Ty, 2);
> + Value* Tmp13 = Builder.CreateShl(Q_3, One);
> + Value* Q_4 = Builder.CreateOr(Carry_2, Tmp13);
> + Builder.CreateBr(End);
> +
> + // ; end: ; preds = %loop-exit, %special-cases
> + // ; %q_5 = phi i32 [ %q_4, %loop-exit ], [ %retVal, %special-cases ]
> + // ; ret i32 %q_5
> + Builder.SetInsertPoint(End, End->begin());
> + PHINode* Q_5 = Builder.CreatePHI(I32Ty, 2);
> +
> + // Populate the Phis, since all values have now been created. Our Phis were:
> + // ; %carry_1 = phi i32 [ 0, %preheader ], [ %carry, %do-while ]
> + Carry_1->addIncoming(Zero, Preheader);
> + Carry_1->addIncoming(Carry, DoWhile);
> + // ; %sr_3 = phi i32 [ %sr_1, %preheader ], [ %sr_2, %do-while ]
> + SR_3->addIncoming(SR_1, Preheader);
> + SR_3->addIncoming(SR_2, DoWhile);
> + // ; %r_1 = phi i32 [ %tmp3, %preheader ], [ %r, %do-while ]
> + R_1->addIncoming(Tmp3, Preheader);
> + R_1->addIncoming(R, DoWhile);
> + // ; %q_2 = phi i32 [ %q, %preheader ], [ %q_1, %do-while ]
> + Q_2->addIncoming(Q, Preheader);
> + Q_2->addIncoming(Q_1, DoWhile);
> + // ; %carry_2 = phi i32 [ 0, %bb1 ], [ %carry, %do-while ]
> + Carry_2->addIncoming(Zero, BB1);
> + Carry_2->addIncoming(Carry, DoWhile);
> + // ; %q_3 = phi i32 [ %q, %bb1 ], [ %q_1, %do-while ]
> + Q_3->addIncoming(Q, BB1);
> + Q_3->addIncoming(Q_1, DoWhile);
> + // ; %q_5 = phi i32 [ %q_4, %loop-exit ], [ %retVal, %special-cases ]
> + Q_5->addIncoming(Q_4, LoopExit);
> + Q_5->addIncoming(RetVal, SpecialCases);
> +
> + return Q_5;
> +}
> +
> +bool llvm::expandDivision(BinaryOperator* Div) {
> + assert(Div->getOpcode() == Instruction::SDiv ||
> + Div->getOpcode() == Instruction::UDiv
> + && "Trying to expand division from a non-division function");
> +
> + IRBuilder<> Builder(Div);
> +
> + if (Div->getType()->isVectorTy()) {
> + assert(0 && "Div over vectors not supported");
> + return false;
> + }
> +
> + // First prepare the sign if it's a signed division
> + if (Div->getOpcode() == Instruction::SDiv) {
> + // Lower the code to unsigned division, and reset Div to point to the udiv.
> + Value* Quotient = GenerateSignedDivisionCode(Div->getOperand(0),
> + Div->getOperand(1), Builder);
> + Div->replaceAllUsesWith(Quotient);
> + Div->dropAllReferences();
> + Div->eraseFromParent();
> +
> + // If we didn't actually generate a udiv instruction, we're done
> + BinaryOperator* BO = dyn_cast<BinaryOperator>(Builder.GetInsertPoint());
> + if (!BO || BO->getOpcode() != Instruction::UDiv)
> + return true;
> +
> + Div = BO;
> + }
> +
> + // Insert the unsigned division code
> + Value* Quotient = GenerateUnsignedDivisionCode(Div->getOperand(0),
> + Div->getOperand(1),
> + Builder);
> + Div->replaceAllUsesWith(Quotient);
> + Div->dropAllReferences();
> + Div->eraseFromParent();
> +
> + return true;
> +}
>
>
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