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Fold subtracts into integer compares vs. zero. This improves generate code for this case on X86

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from
_foo:
        movl    $99, %ecx
        movl    4(%esp), %eax
        subl    %eax, %ecx
        xorl    %edx, %edx
        testl   %ecx, %ecx
        cmovs   %edx, %eax
        ret

to
_foo:
        xorl    %ecx, %ecx
        movl    4(%esp), %eax
        cmpl    $99, %eax
        cmovg   %ecx, %eax
        ret

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@45173 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Christopher Lamb 2007-12-18 21:32:20 +00:00
parent feb8893d17
commit 7a0678cae6
1 changed files with 18 additions and 1 deletions
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19
lib/Transforms/Scalar/InstructionCombining.cpp
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@ -4793,7 +4793,24 @@ Instruction *InstCombiner::visitICmpInst(ICmpInst &I) {
if (isa<UndefValue>(Op1)) // X icmp undef -> undef
return ReplaceInstUsesWith(I, UndefValue::get(Type::Int1Ty));
// (icmp cond (sub m A) 0) ->
// (icmp cond m A)
{
ConstantInt *C1, *C2;
Value *A;
// Check both arguments of the compare for a matching subtract.
if (match(Op0, m_ConstantInt(C1)) && C1->getValue() == 0 &&
match(Op1, m_Sub(m_ConstantInt(C2), m_Value(A)))) {
// We managed to fold the add into the RHS of the select condition.
return new ICmpInst(I.getPredicate(), A, C2);
} else if (match(Op1, m_ConstantInt(C1)) && C1->getValue() == 0 &&
match(Op0, m_Sub(m_ConstantInt(C2), m_Value(A)))) {
// We managed to fold the add into the LHS of the select condition.
return new ICmpInst(I.getPredicate(), C2, A);
}
}
// icmp <global/alloca*/null>, <global/alloca*/null> - Global/Stack value
// addresses never equal each other! We already know that Op0 != Op1.
if ((isa<GlobalValue>(Op0) || isa<AllocaInst>(Op0) ||