[llvm-dev] Automatically scaled offset Load/stores for arrays

[Tim Northover via llvm-dev]

On 6 July 2016 at 10:36, Dilan Manatunga via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
> I have a question on how I would support load/store instructions where the
> offset is automatically scaled by the type. Simply put, any array index
> would be scaled by the width, so that there no longer needs to be a separate.

This is a reasonably common pattern. Typically earlier LLVM passes
(Loop Strength Reduction in particular) wrangle equivalent induction
variables and offsets into what's best for your machine (using
callbacks like TargetTransformInfo::getScalingFactorCost by the looks
of it).

In this case, I'd expect that after setting scale-4 to free and
scale-1 to expensive it will produce something like

    %ptr = %arr_ptr + 4 * %i
    %a = load i32 %ptr
    %x = %x + %a
    %i = %i + 1
    loop!

At that point you have the much simpler task of looking for patterns
like "(load (add $base, (mul $offset, 4)))" during ISel. Quite a few
other targets do this (they usually find it's actually simpler to do
it in C++ using ComplexPatterns, see AArch64's "SelectAddrModeXRO"
functions for example).

Let me know if I've been unclear anywhere.

Tim.