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It is illegal to transform (sdiv (ashr X c1) c2) -> (sdiv x (2^c1 * c2)),

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because C always rounds towards zero.

Thanks Dirk and Ben.

llvm-svn: 162899
This commit is contained in:
Nadav Rotem 2012-08-30 11:23:20 +00:00
parent 627f946e05
commit 5848719c42
2 changed files with 0 additions and 35 deletions
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10
lib/Transforms/InstCombine/InstCombineMulDivRem.cpp
View File

@ -543,16 +543,6 @@ Instruction *InstCombiner::visitSDiv(BinaryOperator &I) {
ConstantExpr::getNeg(RHS));
}
// Sdiv ((Ashl x, C1) , C2) -> x / (C2 * 1<<C1);
if (ConstantInt *C2 = dyn_cast<ConstantInt>(Op1)) {
Value *X;
ConstantInt *C1;
if (match(Op0, m_AShr(m_Value(X), m_ConstantInt(C1)))) {
APInt NC = C2->getValue().shl(C1->getLimitedValue(C1->getBitWidth()-1));
return BinaryOperator::CreateSDiv(X, Builder->getInt(NC));
}
}
// If the sign bits of both operands are zero (i.e. we can prove they are
// unsigned inputs), turn this into a udiv.
if (I.getType()->isIntegerTy()) {

25
test/Transforms/InstCombine/2012-08-28-udiv_ashl.ll
View File

@ -15,15 +15,6 @@ entry:
ret i32 %div1
}
; CHECK: @sdiv400
; CHECK: sdiv i32 %x, 400
; CHECK: ret
define i32 @sdiv400(i32 %x) {
entry:
%div = ashr i32 %x, 2
%div1 = sdiv i32 %div, 100
ret i32 %div1
}
; CHECK: @udiv400_no
; CHECK: ashr
@ -59,22 +50,6 @@ define i80 @udiv_i80(i80 %x) {
ret i80 %div1
}
; CHECK: @sdiv_i80
; CHECK: sdiv i80 %x, 400
; CHECK: ret
define i80 @sdiv_i80(i80 %x) {
%div = ashr i80 %x, 2
%div1 = sdiv i80 %div, 100
ret i80 %div1
}
define i32 @no_crash_notconst_sdiv(i32 %x, i32 %notconst) {
%div = ashr i32 %x, %notconst
%div1 = sdiv i32 %div, 100
ret i32 %div1
}
define i32 @no_crash_notconst_udiv(i32 %x, i32 %notconst) {
%div = lshr i32 %x, %notconst
%div1 = udiv i32 %div, 100