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[AArch64][GlobalISel] Fold constants into G_GLOBAL_VALUE

Authored by paquette on Feb 11 2021, 5:00 PM.

Description

[AArch64][GlobalISel] Fold constants into G_GLOBAL_VALUE

This is pretty much just ports performGlobalAddressCombine from
AArch64ISelLowering. (AArch64 doesn't use the generic DAG combine for this.)

This adds a pre-legalize combine which looks for this pattern:

%g = G_GLOBAL_VALUE @x
%ptr1 = G_PTR_ADD %g, cst1
%ptr2 = G_PTR_ADD %g, cst2
...
%ptrN = G_PTR_ADD %g, cstN

And then, if possible, transforms it like so:

%g = G_GLOBAL_VALUE @x
%offset_g = G_PTR_ADD %g, -min(cst)
%ptr1 = G_PTR_ADD %offset_g, cst1
%ptr2 = G_PTR_ADD %offset_g, cst2
...
%ptrN = G_PTR_ADD %offset_g, cstN

Where min(cst) is the smallest out of the G_PTR_ADD constants.

This means we should save at least one G_PTR_ADD.

This also updates code in the legalizer + selector which assumes that
G_GLOBAL_VALUE will never have an offset and adds/updates relevant tests.

Differential Revision: https://reviews.llvm.org/D96624