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[DivRemPairs] Handling for expanded-form rem - recomposition (PR42673) 

Summary:
While `-div-rem-pairs` pass can decompose rem in div+rem pair when div-rem pair
is unsupported by target, nothing performs the opposite fold.
We can't do that in InstCombine or DAGCombine since neither of those has access to TTI.
So it makes most sense to teach `-div-rem-pairs` about it.

If we matched rem in expanded form, we know we will be able to place div-rem pair
next to each other so we won't regress the situation.
Also, we shouldn't decompose rem if we matched already-decomposed form.
This is surprisingly straight-forward otherwise.

https://bugs.llvm.org/show_bug.cgi?id=42673

Reviewers: spatel, RKSimon, efriedma, ZaMaZaN4iK, bogner

Reviewed By: bogner

Subscribers: bogner, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D65298

llvm-svn: 367288
This commit is contained in:
Roman Lebedev 2019-07-30 07:10:00 +00:00
parent 6606815993
commit c75cdd056f
3 changed files with 103 additions and 17 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

2
llvm/include/llvm/Transforms/Utils/BypassSlowDivision.h
View File

@ -31,6 +31,8 @@ struct DivRemMapKey {
Value *Dividend;
Value *Divisor;
DivRemMapKey() = default;
DivRemMapKey(bool InSignedOp, Value *InDividend, Value *InDivisor)
: SignedOp(InSignedOp), Dividend(InDividend), Divisor(InDivisor) {}
};

110
llvm/lib/Transforms/Scalar/DivRemPairs.cpp
View File

@ -1,4 +1,4 @@
//===- DivRemPairs.cpp - Hoist/decompose division and remainder -*- C++ -*-===//
//===- DivRemPairs.cpp - Hoist/[dr]ecompose division and remainder --------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
@ -6,7 +6,7 @@
//
//===----------------------------------------------------------------------===//
//
// This pass hoists and/or decomposes integer division and remainder
// This pass hoists and/or decomposes/recomposes integer division and remainder
// instructions to enable CFG improvements and better codegen.
//
//===----------------------------------------------------------------------===//
@ -19,19 +19,59 @@
#include "llvm/Analysis/TargetTransformInfo.h"
#include "llvm/IR/Dominators.h"
#include "llvm/IR/Function.h"
#include "llvm/IR/PatternMatch.h"
#include "llvm/Pass.h"
#include "llvm/Support/DebugCounter.h"
#include "llvm/Transforms/Scalar.h"
#include "llvm/Transforms/Utils/BypassSlowDivision.h"
using namespace llvm;
using namespace llvm::PatternMatch;
#define DEBUG_TYPE "div-rem-pairs"
STATISTIC(NumPairs, "Number of div/rem pairs");
STATISTIC(NumRecomposed, "Number of instructions recomposed");
STATISTIC(NumHoisted, "Number of instructions hoisted");
STATISTIC(NumDecomposed, "Number of instructions decomposed");
DEBUG_COUNTER(DRPCounter, "div-rem-pairs-transform",
"Controls transformations in div-rem-pairs pass");
namespace {
using InstructionWithInfo = PointerIntPair<Instruction *, 1, bool /*Expanded*/>;
struct ExpandedMatch {
DivRemMapKey Key;
InstructionWithInfo Value;
};
} // namespace
/// See if we can match: (which is the form we expand into)
/// X - ((X ?/ Y) * Y)
/// which is equivalent to:
/// X ?% Y
static llvm::Optional<ExpandedMatch> matchExpandedRem(Instruction &I) {
ExpandedMatch M;
M.Value.setPointer(&I);
M.Value.setInt(/*Expanded=*/true);
Value *XroundedDownToMultipleOfY;
if (!match(&I, m_Sub(m_Value(M.Key.Dividend),
m_Value(XroundedDownToMultipleOfY))))
return llvm::None;
Instruction *Div;
// Look for ((X / Y) * Y)
if (!match(XroundedDownToMultipleOfY,
m_c_Mul(m_CombineAnd(m_IDiv(m_Specific(M.Key.Dividend),
m_Value(M.Key.Divisor)),
m_Instruction(Div)),
m_Deferred(M.Key.Divisor))))
return llvm::None;
M.Key.SignedOp = Div->getOpcode() == Instruction::SDiv;
return M;
}
/// Find matching pairs of integer div/rem ops (they have the same numerator,
/// denominator, and signedness). If they exist in different basic blocks, bring
/// them together by hoisting or replace the common division operation that is
@ -55,7 +95,7 @@ static bool optimizeDivRem(Function &F, const TargetTransformInfo &TTI,
DenseMap<DivRemMapKey, Instruction *> DivMap;
// Use a MapVector for RemMap so that instructions are moved/inserted in a
// deterministic order.
MapVector<DivRemMapKey, Instruction *> RemMap;
MapVector<DivRemMapKey, InstructionWithInfo> RemMap;
for (auto &BB : F) {
for (auto &I : BB) {
if (I.getOpcode() == Instruction::SDiv)
@ -63,9 +103,13 @@ static bool optimizeDivRem(Function &F, const TargetTransformInfo &TTI,
else if (I.getOpcode() == Instruction::UDiv)
DivMap[DivRemMapKey(false, I.getOperand(0), I.getOperand(1))] = &I;
else if (I.getOpcode() == Instruction::SRem)
RemMap[DivRemMapKey(true, I.getOperand(0), I.getOperand(1))] = &I;
RemMap[DivRemMapKey(true, I.getOperand(0), I.getOperand(1))] = {
&I, /*Expanded=*/false};
else if (I.getOpcode() == Instruction::URem)
RemMap[DivRemMapKey(false, I.getOperand(0), I.getOperand(1))] = &I;
RemMap[DivRemMapKey(false, I.getOperand(0), I.getOperand(1))] = {
&I, /*Expanded=*/false};
else if (auto Match = matchExpandedRem(I))
RemMap[Match->Key] = Match->Value;
}
}
@ -78,13 +122,45 @@ static bool optimizeDivRem(Function &F, const TargetTransformInfo &TTI,
if (!DivInst)
continue;
// We have a matching pair of div/rem instructions. If one dominates the
// other, hoist and/or replace one.
// We have a matching pair of div/rem instructions.
// If the rem is in expanded form, collapse it into rem first.
// If one dominates the other, hoist and/or replace one.
NumPairs++;
Instruction *RemInst = RemPair.second;
bool IsSigned = DivInst->getOpcode() == Instruction::SDiv;
InstructionWithInfo RemLike = RemPair.second;
// NOTE: RemInst may be in expanded form!
Instruction *RemInst = RemLike.getPointer();
bool IsInExpandedForm = RemLike.getInt();
bool IsSigned = RemPair.first.SignedOp;
bool HasDivRemOp = TTI.hasDivRemOp(DivInst->getType(), IsSigned);
if (!DebugCounter::shouldExecute(DRPCounter))
continue;
if (HasDivRemOp && IsInExpandedForm) {
// The target supports div+rem but the rem is expanded.
// We should recompose it first.
Value *X = RemPair.first.Dividend;
Value *Y = RemPair.first.Divisor;
Instruction *RealRem = IsSigned ? BinaryOperator::CreateSRem(X, Y)
: BinaryOperator::CreateURem(X, Y);
// Note that we place it right next to the original expanded instruction,
// and letting further handling to move it if needed.
RealRem->takeName(RemInst);
RealRem->insertAfter(RemInst);
RemInst->replaceAllUsesWith(RealRem);
RemInst->eraseFromParent();
RemInst = RealRem;
NumRecomposed++;
// Note that we have left ((X / Y) * Y) around.
// If it had other uses we could rewrite it as X - X % Y
}
assert(((RemInst->getOpcode() == Instruction::SRem ||
RemInst->getOpcode() == Instruction::URem) ||
!HasDivRemOp) &&
"*If* the target supports div-rem, then by now the RemInst *is* "
"Instruction::[US]Rem.");
// If the target supports div+rem and the instructions are in the same block
// already, there's nothing to do. The backend should handle this. If the
// target does not support div+rem, then we will decompose the rem.
@ -92,10 +168,17 @@ static bool optimizeDivRem(Function &F, const TargetTransformInfo &TTI,
continue;
bool DivDominates = DT.dominates(DivInst, RemInst);
if (!DivDominates && !DT.dominates(RemInst, DivInst))
if (!DivDominates && !DT.dominates(RemInst, DivInst)) {
// We have matching div-rem pair, but they are in two different blocks,
// neither of which dominates one another.
assert(!IsInExpandedForm && "Won't happen if matched expanded form.");
// FIXME: We could hoist both ops to the common predecessor block?
continue;
}
if (!DebugCounter::shouldExecute(DRPCounter))
// The target does not have a single div/rem operation,
// and the rem is already in expanded form. Nothing to do.
if (!HasDivRemOp && IsInExpandedForm)
continue;
if (HasDivRemOp) {
@ -107,8 +190,9 @@ static bool optimizeDivRem(Function &F, const TargetTransformInfo &TTI,
DivInst->moveAfter(RemInst);
NumHoisted++;
} else {
// The target does not have a single div/rem operation. Decompose the
// remainder calculation as:
// The target does not have a single div/rem operation,
// and the rem is *not* in a already-expanded form.
// Decompose the remainder calculation as:
// X % Y --> X - ((X / Y) * Y).
Value *X = RemInst->getOperand(0);
Value *Y = RemInst->getOperand(1);

8
llvm/test/Transforms/DivRemPairs/X86/div-expanded-rem-pair.ll
View File

@ -7,7 +7,7 @@ define void @decompose_illegal_srem_same_block(i32 %a, i32 %b) {
; CHECK-LABEL: @decompose_illegal_srem_same_block(
; CHECK-NEXT: [[DIV:%.*]] = sdiv i32 [[A:%.*]], [[B:%.*]]
; CHECK-NEXT: [[T0:%.*]] = mul i32 [[DIV]], [[B]]
; CHECK-NEXT: [[REM:%.*]] = sub i32 [[A]], [[T0]]
; CHECK-NEXT: [[REM:%.*]] = srem i32 [[A]], [[B]]
; CHECK-NEXT: call void @foo(i32 [[REM]], i32 [[DIV]])
; CHECK-NEXT: ret void
;
@ -22,7 +22,7 @@ define void @decompose_illegal_urem_same_block(i32 %a, i32 %b) {
; CHECK-LABEL: @decompose_illegal_urem_same_block(
; CHECK-NEXT: [[DIV:%.*]] = udiv i32 [[A:%.*]], [[B:%.*]]
; CHECK-NEXT: [[T0:%.*]] = mul i32 [[DIV]], [[B]]
; CHECK-NEXT: [[REM:%.*]] = sub i32 [[A]], [[T0]]
; CHECK-NEXT: [[REM:%.*]] = urem i32 [[A]], [[B]]
; CHECK-NEXT: call void @foo(i32 [[REM]], i32 [[DIV]])
; CHECK-NEXT: ret void
;
@ -39,11 +39,11 @@ define i16 @hoist_srem(i16 %a, i16 %b) {
; CHECK-LABEL: @hoist_srem(
; CHECK-NEXT: entry:
; CHECK-NEXT: [[DIV:%.*]] = sdiv i16 [[A:%.*]], [[B:%.*]]
; CHECK-NEXT: [[REM:%.*]] = srem i16 [[A]], [[B]]
; CHECK-NEXT: [[CMP:%.*]] = icmp eq i16 [[DIV]], 42
; CHECK-NEXT: br i1 [[CMP]], label [[IF:%.*]], label [[END:%.*]]
; CHECK: if:
; CHECK-NEXT: [[T0:%.*]] = mul i16 [[DIV]], [[B]]
; CHECK-NEXT: [[REM:%.*]] = sub i16 [[A]], [[T0]]
; CHECK-NEXT: br label [[END]]
; CHECK: end:
; CHECK-NEXT: [[RET:%.*]] = phi i16 [ [[REM]], [[IF]] ], [ 3, [[ENTRY:%.*]] ]
@ -70,11 +70,11 @@ define i8 @hoist_urem(i8 %a, i8 %b) {
; CHECK-LABEL: @hoist_urem(
; CHECK-NEXT: entry:
; CHECK-NEXT: [[DIV:%.*]] = udiv i8 [[A:%.*]], [[B:%.*]]
; CHECK-NEXT: [[REM:%.*]] = urem i8 [[A]], [[B]]
; CHECK-NEXT: [[CMP:%.*]] = icmp eq i8 [[DIV]], 42
; CHECK-NEXT: br i1 [[CMP]], label [[IF:%.*]], label [[END:%.*]]
; CHECK: if:
; CHECK-NEXT: [[T0:%.*]] = mul i8 [[DIV]], [[B]]
; CHECK-NEXT: [[REM:%.*]] = sub i8 [[A]], [[T0]]
; CHECK-NEXT: br label [[END]]
; CHECK: end:
; CHECK-NEXT: [[RET:%.*]] = phi i8 [ [[REM]], [[IF]] ], [ 3, [[ENTRY:%.*]] ]