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[BypassSlowDivision] Improve our handling of divisions by constants

Browse Source 
(This reapplies r314253.  r314253 was reverted on r314482 because of a
correctness regression on P100, but that regression was identified to be
something else.)

Summary:
Don't bail out on constant divisors for divisions that can be narrowed without
introducing control flow .  This gives us a 32 bit multiply instead of an
emulated 64 bit multiply in the generated PTX assembly.

Reviewers: jlebar

Subscribers: jholewinski, mcrosier, llvm-commits

Differential Revision: https://reviews.llvm.org/D38265

llvm-svn: 319677
This commit is contained in:
Sanjoy Das 2017-12-04 19:21:58 +00:00
parent 17255ac179
commit 7d237893d7
2 changed files with 90 additions and 7 deletions
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20
lib/Transforms/Utils/BypassSlowDivision.cpp
View File

@ -352,11 +352,6 @@ Optional<QuotRemPair> FastDivInsertionTask::insertFastDivAndRem() {
Value *Dividend = SlowDivOrRem->getOperand(0);
Value *Divisor = SlowDivOrRem->getOperand(1);
if (isa<ConstantInt>(Divisor)) {
// Keep division by a constant for DAGCombiner.
return None;
}
VisitedSetTy SetL;
ValueRange DividendRange = getValueRange(Dividend, SetL);
if (DividendRange == VALRNG_LIKELY_LONG)
@ -372,7 +367,9 @@ Optional<QuotRemPair> FastDivInsertionTask::insertFastDivAndRem() {
if (DividendShort && DivisorShort) {
// If both operands are known to be short then just replace the long
// division with a short one in-place.
// division with a short one in-place. Since we're not introducing control
// flow in this case, narrowing the division is always a win, even if the
// divisor is a constant (and will later get replaced by a multiplication).
IRBuilder<> Builder(SlowDivOrRem);
Value *TruncDividend = Builder.CreateTrunc(Dividend, BypassType);
@ -382,7 +379,16 @@ Optional<QuotRemPair> FastDivInsertionTask::insertFastDivAndRem() {
Value *ExtDiv = Builder.CreateZExt(TruncDiv, getSlowType());
Value *ExtRem = Builder.CreateZExt(TruncRem, getSlowType());
return QuotRemPair(ExtDiv, ExtRem);
} else if (DividendShort && !isSignedOp()) {
}
if (isa<ConstantInt>(Divisor)) {
// If the divisor is not a constant, DAGCombiner will convert it to a
// multiplication by a magic constant. It isn't clear if it is worth
// introducing control flow to get a narrower multiply.
return None;
}
if (DividendShort && !isSignedOp()) {
// If the division is unsigned and Dividend is known to be short, then
// either
// 1) Divisor is less or equal to Dividend, and the result can be computed

77
test/Transforms/CodeGenPrepare/NVPTX/bypass-slow-div.ll
View File

@ -27,3 +27,80 @@ define void @rem_only(i64 %a, i64 %b, i64* %retptr) {
store i64 %d, i64* %retptr
ret void
}
; CHECK-LABEL: @udiv_by_constant(
define i64 @udiv_by_constant(i32 %a) {
; CHECK-NEXT: [[A_ZEXT:%.*]] = zext i32 [[A:%.*]] to i64
; CHECK-NEXT: [[TMP1:%.*]] = trunc i64 [[A_ZEXT]] to i32
; CHECK-NEXT: [[TMP2:%.*]] = udiv i32 [[TMP1]], 50
; CHECK-NEXT: [[TMP3:%.*]] = zext i32 [[TMP2]] to i64
; CHECK-NEXT: ret i64 [[TMP3]]
%a.zext = zext i32 %a to i64
%wide.div = udiv i64 %a.zext, 50
ret i64 %wide.div
}
; CHECK-LABEL: @urem_by_constant(
define i64 @urem_by_constant(i32 %a) {
; CHECK-NEXT: [[A_ZEXT:%.*]] = zext i32 [[A:%.*]] to i64
; CHECK-NEXT: [[TMP1:%.*]] = trunc i64 [[A_ZEXT]] to i32
; CHECK-NEXT: [[TMP2:%.*]] = urem i32 [[TMP1]], 50
; CHECK-NEXT: [[TMP3:%.*]] = zext i32 [[TMP2]] to i64
; CHECK-NEXT: ret i64 [[TMP3]]
%a.zext = zext i32 %a to i64
%wide.div = urem i64 %a.zext, 50
ret i64 %wide.div
}
; Negative test: instead of emitting a runtime check on %a, we prefer to let the
; DAGCombiner transform this division by constant into a multiplication (with a
; "magic constant").
;
; CHECK-LABEL: @udiv_by_constant_negative_0(
define i64 @udiv_by_constant_negative_0(i64 %a) {
; CHECK-NEXT: [[WIDE_DIV:%.*]] = udiv i64 [[A:%.*]], 50
; CHECK-NEXT: ret i64 [[WIDE_DIV]]
%wide.div = udiv i64 %a, 50
ret i64 %wide.div
}
; Negative test: while we know the dividend is short, the divisor isn't. This
; test is here for completeness, but instcombine will optimize this to return 0.
;
; CHECK-LABEL: @udiv_by_constant_negative_1(
define i64 @udiv_by_constant_negative_1(i32 %a) {
; CHECK-NEXT: [[A_ZEXT:%.*]] = zext i32 [[A:%.*]] to i64
; CHECK-NEXT: [[WIDE_DIV:%.*]] = udiv i64 [[A_ZEXT]], 8589934592
; CHECK-NEXT: ret i64 [[WIDE_DIV]]
%a.zext = zext i32 %a to i64
%wide.div = udiv i64 %a.zext, 8589934592 ;; == 1 << 33
ret i64 %wide.div
}
; URem version of udiv_by_constant_negative_0
;
; CHECK-LABEL: @urem_by_constant_negative_0(
define i64 @urem_by_constant_negative_0(i64 %a) {
; CHECK-NEXT: [[WIDE_DIV:%.*]] = urem i64 [[A:%.*]], 50
; CHECK-NEXT: ret i64 [[WIDE_DIV]]
%wide.div = urem i64 %a, 50
ret i64 %wide.div
}
; URem version of udiv_by_constant_negative_1
;
; CHECK-LABEL: @urem_by_constant_negative_1(
define i64 @urem_by_constant_negative_1(i32 %a) {
; CHECK-NEXT: [[A_ZEXT:%.*]] = zext i32 [[A:%.*]] to i64
; CHECK-NEXT: [[WIDE_DIV:%.*]] = urem i64 [[A_ZEXT]], 8589934592
; CHECK-NEXT: ret i64 [[WIDE_DIV]]
%a.zext = zext i32 %a to i64
%wide.div = urem i64 %a.zext, 8589934592 ;; == 1 << 33
ret i64 %wide.div
}