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Fold certain additions through selects (and their compares) so as to eliminate subtractions. This code is often produced by the SMAX expansion in SCEV.

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This implements test/Transforms/InstCombine/2007-12-18-AddSelCmpSub.ll

llvm-svn: 45158
This commit is contained in:
Christopher Lamb 2007-12-18 09:34:41 +00:00
parent 36bfae49e3
commit aeb76743dc
3 changed files with 99 additions and 18 deletions
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18
lib/Target/README.txt
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@ -464,21 +464,3 @@ entry:
}
//===---------------------------------------------------------------------===//
This code is often produced by the SMAX expansion in SCEV:
define i32 @foo(i32 %a) {
entry:
%tmp15 = sub i32 99, %a ; <i32> [#uses=2]
%tmp16 = icmp slt i32 %tmp15, 0 ; <i1> [#uses=1]
%smax = select i1 %tmp16, i32 0, i32 %tmp15 ; <i32> [#uses=1]
%tmp12 = add i32 %smax, %a ; <i32> [#uses=1]
%tmp13 = add i32 %tmp12, 1 ; <i32> [#uses=1]
ret i32 %tmp13
}
Note that the tmp12 add can be pushed through the select operands, turning
it into a "select %tmp16, %a, 99". We apparently already do this in dag
combine because it isn't present in X86 output.
//===---------------------------------------------------------------------===//

61
lib/Transforms/Scalar/InstructionCombining.cpp
View File

@ -2130,6 +2130,67 @@ Instruction *InstCombiner::visitAdd(BinaryOperator &I) {
return new PtrToIntInst(I2, CI->getType());
}
}
// add (select (icmp 0 (sub m A)) X Y) A ->
// add (select (icmp A m) X Y) A
//
// add (select X 0 (sub n A)) A ->
// select X A n ->
{
SelectInst *SI = dyn_cast<SelectInst>(LHS);
Value *Other = RHS;
if (!SI) {
SI = dyn_cast<SelectInst>(RHS);
Other = LHS;
}
if (SI) {
Value *TV = SI->getTrueValue();
Value *FV = SI->getFalseValue();
Value *A;
// Can we fold the add into the argument of the compare?
Value *Cond = SI->getCondition();
if (ICmpInst *IC = dyn_cast<ICmpInst>(Cond)) {
Value *ICOp0 = IC->getOperand(0);
Value *ICOp1 = IC->getOperand(1);
ConstantInt *C3, *C4;
// Check both arguments of the compare for a matching subtract.
if (match(ICOp0, m_ConstantInt(C3)) && C3->getValue() == 0 &&
match(ICOp1, m_Sub(m_ConstantInt(C4), m_Value(A))) &&
A == Other) {
// We managed to fold the add into the RHS of the select condition.
Cond = new ICmpInst(IC->getPredicate(), A, C4, "asis", SI);
} else if (match(ICOp1, m_ConstantInt(C3)) && C3->getValue() == 0 &&
match(ICOp0, m_Sub(m_ConstantInt(C4), m_Value(A))) &&
A == Other) {
// We managed to fold the add into the LHS of the select condition.
Cond = new ICmpInst(IC->getPredicate(), C4, A, "asis", SI);
}
}
// Can we fold the add into the argument of the select?
// We check both true and false select arguments for a matching subtract.
ConstantInt *C1, *C2;
if (match(FV, m_ConstantInt(C1)) && C1->getValue() == 0 &&
match(TV, m_Sub(m_ConstantInt(C2), m_Value(A))) &&
A == Other) {
// We managed to fold the add into the true select value,
// picking up a simplified condition, if available.
return new SelectInst(Cond, C2, A, "adselsub");
} else if (match(TV, m_ConstantInt(C1)) && C1->getValue() == 0 &&
match(FV, m_Sub(m_ConstantInt(C2), m_Value(A))) &&
A == Other) {
// We managed to fold the add into the false select value,
// picking up a simplified condition, if available.
return new SelectInst(Cond, A, C2, "adselsub");
} else if (Cond != SI->getCondition()) {
// We only managed to fold the add into the select condition.
SI->setOperand(0, Cond);
Changed = true;
}
}
}
return Changed ? &I : 0;
}

38
test/Transforms/InstCombine/2007-12-18-AddSelCmpSub.ll Normal file
View File

@ -0,0 +1,38 @@
; RUN: llvm-as < %s | opt -instcombine | llvm-dis | not grep {sub}
define i32 @foo(i32 %a) {
entry:
%tmp15 = sub i32 99, %a ; <i32> [#uses=2]
%tmp16 = icmp slt i32 %tmp15, 0 ; <i1> [#uses=1]
%smax = select i1 %tmp16, i32 0, i32 %tmp15 ; <i32> [#uses=1]
%tmp12 = add i32 %smax, %a ; <i32> [#uses=1]
%tmp13 = add i32 %tmp12, 1 ; <i32> [#uses=1]
ret i32 %tmp13
}
define i32 @bar(i32 %a) {
entry:
%tmp15 = sub i32 99, %a ; <i32> [#uses=2]
%tmp16 = icmp slt i32 %tmp15, 0 ; <i1> [#uses=1]
%smax = select i1 %tmp16, i32 0, i32 %tmp15 ; <i32> [#uses=1]
%tmp12 = add i32 %smax, %a ; <i32> [#uses=1]
ret i32 %tmp12
}
define i32 @baz(i32 %a) {
entry:
%tmp15 = sub i32 99, %a ; <i32> [#uses=1]
%tmp16 = icmp slt i32 %tmp15, 0 ; <i1> [#uses=1]
%smax = select i1 %tmp16, i32 0, i32 42 ; <i32> [#uses=1]
%tmp12 = add i32 %smax, %a ; <i32> [#uses=1]
ret i32 %tmp12
}
define i32 @fun(i32 %a) {
entry:
%tmp15 = sub i32 99, %a ; <i32> [#uses=1]
%tmp16 = icmp slt i32 %a, 0 ; <i1> [#uses=1]
%smax = select i1 %tmp16, i32 0, i32 %tmp15 ; <i32> [#uses=1]
%tmp12 = add i32 %smax, %a ; <i32> [#uses=1]
ret i32 %tmp12
}