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[InstSimplify] use knownbits to fold more udiv/urem 

We could use knownbits on both operands for even more folds (and there are
already tests in place for that), but this is enough to recover the example
from:
https://github.com/llvm/llvm-project/issues/51934
(the tests are derived from the code in that example)

I am assuming no noticeable compile-time impact from this because udiv/urem
are rare opcodes.

Differential Revision: https://reviews.llvm.org/D116616
This commit is contained in:
Sanjay Patel 2022-01-12 14:58:24 -05:00
parent 56f62fbf73
commit 6bd127b079
3 changed files with 16 additions and 6 deletions
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10
llvm/lib/Analysis/InstructionSimplify.cpp
View File

@ -1079,6 +1079,16 @@ static bool isDivZero(Value *X, Value *Y, const SimplifyQuery &Q,
}
// IsSigned == false.
// Is the unsigned dividend known to be less than a constant divisor?
// TODO: Convert this (and above) to range analysis
// ("computeConstantRangeIncludingKnownBits")?
const APInt *C;
if (match(Y, m_APInt(C)) &&
computeKnownBits(X, Q.DL, 0, Q.AC, Q.CxtI, Q.DT).getMaxValue().ult(*C))
return true;
// Try again for any divisor:
// Is the dividend unsigned less than the divisor?
return isICmpTrue(ICmpInst::ICMP_ULT, X, Y, Q, MaxRecurse);
}

7
llvm/test/Transforms/InstSimplify/div.ll
View File

@ -155,10 +155,7 @@ define i32 @not_udiv_constant_dividend_known_smaller_than_divisor(i32 %x) {
define i8 @udiv_dividend_known_smaller_than_constant_divisor2(i1 %b) {
; CHECK-LABEL: @udiv_dividend_known_smaller_than_constant_divisor2(
; CHECK-NEXT: [[T0:%.*]] = zext i1 [[B:%.*]] to i8
; CHECK-NEXT: [[XOR:%.*]] = xor i8 [[T0]], 12
; CHECK-NEXT: [[R:%.*]] = udiv i8 [[XOR]], 14
; CHECK-NEXT: ret i8 [[R]]
; CHECK-NEXT: ret i8 0
;
%t0 = zext i1 %b to i8
%xor = xor i8 %t0, 12
@ -166,6 +163,8 @@ define i8 @udiv_dividend_known_smaller_than_constant_divisor2(i1 %b) {
ret i8 %r
}
; negative test - dividend can equal 13
define i8 @not_udiv_dividend_known_smaller_than_constant_divisor2(i1 %b) {
; CHECK-LABEL: @not_udiv_dividend_known_smaller_than_constant_divisor2(
; CHECK-NEXT: [[T0:%.*]] = zext i1 [[B:%.*]] to i8

5
llvm/test/Transforms/InstSimplify/rem.ll
View File

@ -187,8 +187,7 @@ define i8 @urem_dividend_known_smaller_than_constant_divisor2(i1 %b) {
; CHECK-LABEL: @urem_dividend_known_smaller_than_constant_divisor2(
; CHECK-NEXT: [[T0:%.*]] = zext i1 [[B:%.*]] to i8
; CHECK-NEXT: [[XOR:%.*]] = xor i8 [[T0]], 12
; CHECK-NEXT: [[R:%.*]] = urem i8 [[XOR]], 14
; CHECK-NEXT: ret i8 [[R]]
; CHECK-NEXT: ret i8 [[XOR]]
;
%t0 = zext i1 %b to i8
%xor = xor i8 %t0, 12
@ -196,6 +195,8 @@ define i8 @urem_dividend_known_smaller_than_constant_divisor2(i1 %b) {
ret i8 %r
}
; negative test - dividend can equal 13
define i8 @not_urem_dividend_known_smaller_than_constant_divisor2(i1 %b) {
; CHECK-LABEL: @not_urem_dividend_known_smaller_than_constant_divisor2(
; CHECK-NEXT: [[T0:%.*]] = zext i1 [[B:%.*]] to i8