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Transform any logical shift of a power of two into an exact/NUW shift when 

in a known-non-zero context.

llvm-svn: 131887
This commit is contained in:
Chris Lattner 2011-05-23 00:21:50 +00:00
parent f5199debc0
commit 8aff4f8efc
2 changed files with 38 additions and 0 deletions
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17
llvm/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp
View File

@ -29,6 +29,23 @@ static Value *simplifyValueKnownNonZero(Value *V, InstCombiner &IC) {
// code.
if (!V->hasOneUse()) return 0;
// (PowerOfTwo >>u B) --> isExact since shifting out the result would make it
// inexact. Similarly for <<.
if (BinaryOperator *I = dyn_cast<BinaryOperator>(V))
if (I->isLogicalShift() &&
isPowerOfTwo(I->getOperand(0), IC.getTargetData())) {
if (I->getOpcode() == Instruction::LShr && !I->isExact()) {
I->setIsExact();
return I;
}
if (I->getOpcode() == Instruction::Shl && !I->hasNoUnsignedWrap()) {
I->setHasNoUnsignedWrap();
return I;
}
}
// ((1 << A) >>u B) --> (1 << (A-B))
// Because V cannot be zero, we know that B is less than A.
Value *A = 0, *B = 0, *PowerOf2 = 0;

21
llvm/test/Transforms/InstCombine/shift.ll
View File

@ -506,3 +506,24 @@ define i32 @test41(i32 %a, i32 %b) nounwind {
; CHECK-NEXT: shl i32 8, %b
; CHECK-NEXT: ret i32
}
define i32 @test42(i32 %a, i32 %b) nounwind {
%div = lshr i32 4096, %b ; must be exact otherwise we'd divide by zero
%div2 = udiv i32 %a, %div
ret i32 %div2
; CHECK: @test42
; CHECK-NEXT: lshr exact i32 4096, %b
}
define i32 @test43(i32 %a, i32 %b) nounwind {
%div = shl i32 4096, %b ; must be exact otherwise we'd divide by zero
%div2 = udiv i32 %a, %div
ret i32 %div2
; CHECK: @test43
; CHECK-NEXT: add i32 %b, 12
; CHECK-NEXT: lshr
; CHECK-NEXT: ret
}