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[CGP] [CodeGenPrepare] Folding urem with loop invariant value plus offset #104724

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merged 1 commit into from
Oct 31, 2024
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[CGP] [CodeGenPrepare] Folding urem with loop invariant value plus offset #104724

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58 changes: 49 additions & 9 deletions llvm/lib/CodeGen/CodeGenPrepare.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -1982,17 +1982,36 @@ static bool foldFCmpToFPClassTest(CmpInst *Cmp, const TargetLowering &TLI,
return true;
}

static bool isRemOfLoopIncrementWithLoopInvariant(Instruction *Rem,
const LoopInfo *LI,
Value *&RemAmtOut,
PHINode *&LoopIncrPNOut) {
static bool isRemOfLoopIncrementWithLoopInvariant(
Instruction *Rem, const LoopInfo *LI, Value *&RemAmtOut, Value *&AddInstOut,
Value *&AddOffsetOut, PHINode *&LoopIncrPNOut) {
Value *Incr, *RemAmt;
// NB: If RemAmt is a power of 2 it *should* have been transformed by now.
if (!match(Rem, m_URem(m_Value(Incr), m_Value(RemAmt))))
return false;

Value *AddInst, *AddOffset;
// Find out loop increment PHI.
auto *PN = dyn_cast<PHINode>(Incr);
if (PN != nullptr) {
AddInst = nullptr;
AddOffset = nullptr;
} else {
// Search through a NUW add on top of the loop increment.
Value *V0, *V1;
if (!match(Incr, m_NUWAdd(m_Value(V0), m_Value(V1))))
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@RKSimon RKSimon Oct 10, 2024

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Would it be worth adding a m_PHI matcher - we could then use m_c_NUWAdd and avoid the messy commutative matching below.

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@goldsteinn goldsteinn Oct 10, 2024

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We could, although Im not sure exactly what we would want in a PHI matcher? Would we support matching BBs? Variable number of arguments or "binary" phis only? Etc...
Either way, I think that its a conversation that is a bit beyond the scope of this particular use case and PR.

return false;

AddInst = Incr;
PN = dyn_cast<PHINode>(V0);
if (PN != nullptr) {
AddOffset = V1;
} else {
PN = dyn_cast<PHINode>(V1);
AddOffset = V0;
}
}

if (!PN)
return false;

Expand Down Expand Up @@ -2032,6 +2051,8 @@ static bool isRemOfLoopIncrementWithLoopInvariant(Instruction *Rem,
// Set output variables.
RemAmtOut = RemAmt;
LoopIncrPNOut = PN;
AddInstOut = AddInst;
AddOffsetOut = AddOffset;

return true;
}
Expand All @@ -2046,15 +2067,14 @@ static bool isRemOfLoopIncrementWithLoopInvariant(Instruction *Rem,
// Rem = (Start nuw+ IncrLoopInvariant) % RemAmtLoopInvariant;
// for(i = Start; i < End; ++i, ++rem)
// Rem = rem == RemAmtLoopInvariant ? 0 : Rem;
//
// Currently only implemented for `IncrLoopInvariant` being zero.
static bool foldURemOfLoopIncrement(Instruction *Rem, const DataLayout *DL,
const LoopInfo *LI,
SmallSet<BasicBlock *, 32> &FreshBBs,
bool IsHuge) {
Value *RemAmt;
Value *AddOffset, *RemAmt, *AddInst;
PHINode *LoopIncrPN;
if (!isRemOfLoopIncrementWithLoopInvariant(Rem, LI, RemAmt, LoopIncrPN))
if (!isRemOfLoopIncrementWithLoopInvariant(Rem, LI, RemAmt, AddInst,
AddOffset, LoopIncrPN))
return false;

// Only non-constant remainder as the extra IV is probably not profitable
Expand All @@ -2072,6 +2092,23 @@ static bool foldURemOfLoopIncrement(Instruction *Rem, const DataLayout *DL,

Loop *L = LI->getLoopFor(LoopIncrPN->getParent());
Value *Start = LoopIncrPN->getIncomingValueForBlock(L->getLoopPreheader());
// If we have add create initial value for remainder.
// The logic here is:
// (urem (add nuw Start, IncrLoopInvariant), RemAmtLoopInvariant
//
// Only proceed if the expression simplifies (otherwise we can't fully
// optimize out the urem).
if (AddInst) {
assert(AddOffset && "We found an add but missing values");
// Without dom-condition/assumption cache we aren't likely to get much out
// of a context instruction.
Start = simplifyAddInst(Start, AddOffset,
match(AddInst, m_NSWAdd(m_Value(), m_Value())),
/*IsNUW=*/true, *DL);
if (!Start)
return false;
}

// If we can't fully optimize out the `rem`, skip this transform.
Start = simplifyURemInst(Start, RemAmt, *DL);
if (!Start)
Expand Down Expand Up @@ -2099,9 +2136,12 @@ static bool foldURemOfLoopIncrement(Instruction *Rem, const DataLayout *DL,
FreshBBs.insert(LoopIncrPN->getParent());
FreshBBs.insert(L->getLoopLatch());
FreshBBs.insert(Rem->getParent());

if (AddInst)
FreshBBs.insert(cast<Instruction>(AddInst)->getParent());
replaceAllUsesWith(Rem, NewRem, FreshBBs, IsHuge);
Rem->eraseFromParent();
if (AddInst && AddInst->use_empty())
cast<Instruction>(AddInst)->eraseFromParent();
return true;
}

Expand Down
16 changes: 9 additions & 7 deletions llvm/test/Transforms/CodeGenPrepare/X86/fold-loop-of-urem.ll
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Original file line number Diff line number Diff line change
Expand Up @@ -319,20 +319,20 @@ for.body.tail:
define void @simple_urem_to_sel_vec(<2 x i64> %rem_amt) nounwind {
; CHECK-LABEL: define void @simple_urem_to_sel_vec(
; CHECK-SAME: <2 x i64> [[REM_AMT:%.*]]) #[[ATTR0]] {
; CHECK-NEXT: [[FOR_COND_CLEANUP:.*]]:
; CHECK-NEXT: [[ENTRY:.*]]:
; CHECK-NEXT: br label %[[FOR_BODY:.*]]
; CHECK: [[ENTRY:.*]]:
; CHECK: [[FOR_COND_CLEANUP:.*]]:
; CHECK-NEXT: ret void
; CHECK: [[FOR_BODY]]:
; CHECK-NEXT: [[REM:%.*]] = phi <2 x i64> [ zeroinitializer, %[[FOR_COND_CLEANUP]] ], [ [[TMP3:%.*]], %[[FOR_BODY]] ]
; CHECK-NEXT: [[I_04:%.*]] = phi <2 x i64> [ [[INC:%.*]], %[[FOR_BODY]] ], [ zeroinitializer, %[[FOR_COND_CLEANUP]] ]
; CHECK-NEXT: [[REM:%.*]] = phi <2 x i64> [ zeroinitializer, %[[ENTRY]] ], [ [[TMP3:%.*]], %[[FOR_BODY]] ]
; CHECK-NEXT: [[I_04:%.*]] = phi <2 x i64> [ [[INC:%.*]], %[[FOR_BODY]] ], [ zeroinitializer, %[[ENTRY]] ]
; CHECK-NEXT: tail call void @use.2xi64(<2 x i64> [[REM]])
; CHECK-NEXT: [[TMP1:%.*]] = add nuw <2 x i64> [[REM]], <i64 1, i64 1>
; CHECK-NEXT: [[TMP2:%.*]] = icmp eq <2 x i64> [[TMP1]], [[REM_AMT]]
; CHECK-NEXT: [[TMP3]] = select <2 x i1> [[TMP2]], <2 x i64> zeroinitializer, <2 x i64> [[TMP1]]
; CHECK-NEXT: [[INC]] = add nuw <2 x i64> [[I_04]], <i64 1, i64 1>
; CHECK-NEXT: [[EXITCOND_NOT:%.*]] = call i1 @get.i1()
; CHECK-NEXT: br i1 [[EXITCOND_NOT]], label %[[ENTRY]], label %[[FOR_BODY]]
; CHECK-NEXT: br i1 [[EXITCOND_NOT]], label %[[FOR_COND_CLEANUP]], label %[[FOR_BODY]]
;
entry:
br label %for.body
Expand Down Expand Up @@ -892,10 +892,12 @@ define void @simple_urem_to_sel_non_zero_start_through_add(i32 %N, i32 %rem_amt_
; CHECK: [[FOR_COND_CLEANUP]]:
; CHECK-NEXT: ret void
; CHECK: [[FOR_BODY]]:
; CHECK-NEXT: [[REM:%.*]] = phi i32 [ 7, %[[FOR_BODY_PREHEADER]] ], [ [[TMP3:%.*]], %[[FOR_BODY]] ]
; CHECK-NEXT: [[I_04:%.*]] = phi i32 [ [[INC:%.*]], %[[FOR_BODY]] ], [ 2, %[[FOR_BODY_PREHEADER]] ]
; CHECK-NEXT: [[I_WITH_OFF:%.*]] = add nuw i32 [[I_04]], 5
; CHECK-NEXT: [[REM:%.*]] = urem i32 [[I_WITH_OFF]], [[REM_AMT]]
; CHECK-NEXT: tail call void @use.i32(i32 [[REM]])
; CHECK-NEXT: [[TMP1:%.*]] = add nuw i32 [[REM]], 1
; CHECK-NEXT: [[TMP2:%.*]] = icmp eq i32 [[TMP1]], [[REM_AMT]]
; CHECK-NEXT: [[TMP3]] = select i1 [[TMP2]], i32 0, i32 [[TMP1]]
; CHECK-NEXT: [[INC]] = add nuw i32 [[I_04]], 1
; CHECK-NEXT: [[EXITCOND_NOT:%.*]] = icmp eq i32 [[INC]], [[N]]
; CHECK-NEXT: br i1 [[EXITCOND_NOT]], label %[[FOR_COND_CLEANUP]], label %[[FOR_BODY]]
Expand Down