On Dec 18, 2007, at 1:32 PM, Christopher Lamb wrote:
> Fold subtracts into integer compares vs. zero. This improves
> generate code for this case on X86
> from
Very nice xform! Please add a testcase to Transforms/Instcombine
Also:
> 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;
This code can be simplified in two ways: first, you can drop the
first version of the pattern: compares (and other binops, where
possible) are canonicalized so that any constant will be on the RHS.
This means you don't have to test to see if the LHS is a constant.
Second, if you sink this down into this "if" block:
// See if we are doing a comparison between a constant and an
instruction that
// can be folded into the comparison.
if (ConstantInt *CI = dyn_cast<ConstantInt>(Op1)) {
then you can turn the first match into just: if (CI->isNullValue() &&
match(Op0, ...)
Finally, is "m" really required to be a constant? This xform seems
valid for any "m" and "A", and more general is good.
Thanks Christopher!
-Chris
> + // 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) ||
>
>
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