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[DivRemPairs] Add an initial case for hoisting to a common predecessor.

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This patch adds support for hoisting the division and maybe the
remainder for control flow graphs like this.

```
PredBB
  |  \
  |  Rem
  |  /
 Div
```

If we have DivRem we'll hoist both to PredBB. If not we'll just
hoist Div and expand Rem using the Div.

This improves our codegen for something like this

```
__uint128_t udivmodti4(__uint128_t dividend, __uint128_t divisor, __uint128_t *remainder) {
    if (remainder != 0)
        *remainder = dividend % divisor;
    return dividend / divisor;
}
```

Reviewed By: spatel, lebedev.ri

Differential Revision: https://reviews.llvm.org/D87555
This commit is contained in:
Craig Topper 2021-07-11 10:03:05 -07:00 committed by Craig Topper
parent 7051794d25
commit 34eef4175e
2 changed files with 58 additions and 11 deletions
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48
lib/Transforms/Scalar/DivRemPairs.cpp
View File

@ -238,8 +238,52 @@ static bool optimizeDivRem(Function &F, const TargetTransformInfo &TTI,
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.
// FIXME: We could hoist both ops to the common predecessor block?
continue;
BasicBlock *PredBB = nullptr;
BasicBlock *DivBB = DivInst->getParent();
BasicBlock *RemBB = RemInst->getParent();
// It's only safe to hoist if every instruction before the Div/Rem in the
// basic block is guaranteed to transfer execution.
auto IsSafeToHoist = [](Instruction *DivOrRem, BasicBlock *ParentBB) {
for (auto I = ParentBB->begin(), E = DivOrRem->getIterator(); I != E;
++I)
if (!isGuaranteedToTransferExecutionToSuccessor(&*I))
return false;
return true;
};
// Look for something like this
// PredBB
// | \
// | Rem
// | /
// Div
//
// If the Rem block has a single predecessor and successor, and all paths
// from PredBB go to either RemBB or DivBB, and execution of RemBB and
// DivBB will always reach the Div/Rem, we can hoist Div to PredBB. If
// we have a DivRem operation we can also hoist Rem. Otherwise we'll leave
// Rem where it is and rewrite it to mul/sub.
// FIXME: We could handle more hoisting cases.
if (RemBB->getSingleSuccessor() == DivBB)
PredBB = RemBB->getUniquePredecessor();
if (PredBB && IsSafeToHoist(RemInst, RemBB) &&
IsSafeToHoist(DivInst, DivBB) &&
llvm::all_of(successors(PredBB), [&](BasicBlock *BB) {
return BB == DivBB || BB == RemBB;
})) {
DivDominates = true;
DivInst->moveBefore(PredBB->getTerminator());
Changed = true;
if (HasDivRemOp) {
RemInst->moveBefore(PredBB->getTerminator());
continue;
}
} else
continue;
}
// The target does not have a single div/rem operation,

21
test/Transforms/DivRemPairs/X86/div-expanded-rem-pair.ll
View File

@ -210,13 +210,13 @@ define i64 @remainder_triangle_i64(i64 %a, i64 %b, i64* %rp) {
; CHECK-LABEL: @remainder_triangle_i64(
; CHECK-NEXT: entry:
; CHECK-NEXT: [[CMP:%.*]] = icmp ne i64* [[RP:%.*]], null
; CHECK-NEXT: [[DIV:%.*]] = udiv i64 [[A:%.*]], [[B:%.*]]
; CHECK-NEXT: [[REM:%.*]] = urem i64 [[A]], [[B]]
; CHECK-NEXT: br i1 [[CMP]], label [[IF_THEN:%.*]], label [[END:%.*]]
; CHECK: if.then:
; CHECK-NEXT: [[REM:%.*]] = urem i64 [[A:%.*]], [[B:%.*]]
; CHECK-NEXT: store i64 [[REM]], i64* [[RP]], align 4
; CHECK-NEXT: br label [[END]]
; CHECK: end:
; CHECK-NEXT: [[DIV:%.*]] = udiv i64 [[A]], [[B]]
; CHECK-NEXT: ret i64 [[DIV]]
;
entry:
@ -239,13 +239,16 @@ define i128 @remainder_triangle_i128(i128 %a, i128 %b, i128* %rp) {
; CHECK-LABEL: @remainder_triangle_i128(
; CHECK-NEXT: entry:
; CHECK-NEXT: [[CMP:%.*]] = icmp ne i128* [[RP:%.*]], null
; CHECK-NEXT: [[A_FROZEN:%.*]] = freeze i128 [[A:%.*]]
; CHECK-NEXT: [[B_FROZEN:%.*]] = freeze i128 [[B:%.*]]
; CHECK-NEXT: [[DIV:%.*]] = udiv i128 [[A_FROZEN]], [[B_FROZEN]]
; CHECK-NEXT: br i1 [[CMP]], label [[IF_THEN:%.*]], label [[END:%.*]]
; CHECK: if.then:
; CHECK-NEXT: [[REM:%.*]] = urem i128 [[A:%.*]], [[B:%.*]]
; CHECK-NEXT: store i128 [[REM]], i128* [[RP]], align 4
; CHECK-NEXT: [[TMP0:%.*]] = mul i128 [[DIV]], [[B_FROZEN]]
; CHECK-NEXT: [[REM_DECOMPOSED:%.*]] = sub i128 [[A_FROZEN]], [[TMP0]]
; CHECK-NEXT: store i128 [[REM_DECOMPOSED]], i128* [[RP]], align 4
; CHECK-NEXT: br label [[END]]
; CHECK: end:
; CHECK-NEXT: [[DIV:%.*]] = udiv i128 [[A]], [[B]]
; CHECK-NEXT: ret i128 [[DIV]]
;
entry:
@ -265,16 +268,16 @@ end:
define i64 @remainder_triangle_i64_multiple_rem_edges(i64 %a, i64 %b, i64 %c, i64* %rp) {
; CHECK-LABEL: @remainder_triangle_i64_multiple_rem_edges(
; CHECK-NEXT: entry:
; CHECK-NEXT: [[DIV:%.*]] = udiv i64 [[A:%.*]], [[B:%.*]]
; CHECK-NEXT: [[REM:%.*]] = urem i64 [[A]], [[B]]
; CHECK-NEXT: switch i64 [[C:%.*]], label [[SW_DEFAULT:%.*]] [
; CHECK-NEXT: i64 0, label [[SW_BB:%.*]]
; CHECK-NEXT: i64 2, label [[SW_BB]]
; CHECK-NEXT: ]
; CHECK: sw.bb:
; CHECK-NEXT: [[REM:%.*]] = urem i64 [[A:%.*]], [[B:%.*]]
; CHECK-NEXT: store i64 [[REM]], i64* [[RP:%.*]], align 4
; CHECK-NEXT: br label [[SW_DEFAULT]]
; CHECK: sw.default:
; CHECK-NEXT: [[DIV:%.*]] = udiv i64 [[A]], [[B]]
; CHECK-NEXT: ret i64 [[DIV]]
;
entry:
@ -296,16 +299,16 @@ sw.default: ; preds = %entry, %sw.bb
define i64 @remainder_triangle_i64_multiple_div_edges(i64 %a, i64 %b, i64 %c, i64* %rp) {
; CHECK-LABEL: @remainder_triangle_i64_multiple_div_edges(
; CHECK-NEXT: entry:
; CHECK-NEXT: [[DIV:%.*]] = udiv i64 [[A:%.*]], [[B:%.*]]
; CHECK-NEXT: [[REM:%.*]] = urem i64 [[A]], [[B]]
; CHECK-NEXT: switch i64 [[C:%.*]], label [[SW_DEFAULT:%.*]] [
; CHECK-NEXT: i64 0, label [[SW_BB:%.*]]
; CHECK-NEXT: i64 2, label [[SW_BB]]
; CHECK-NEXT: ]
; CHECK: sw.default:
; CHECK-NEXT: [[REM:%.*]] = urem i64 [[A:%.*]], [[B:%.*]]
; CHECK-NEXT: store i64 [[REM]], i64* [[RP:%.*]], align 4
; CHECK-NEXT: br label [[SW_BB]]
; CHECK: sw.bb:
; CHECK-NEXT: [[DIV:%.*]] = udiv i64 [[A]], [[B]]
; CHECK-NEXT: ret i64 [[DIV]]
;
entry: