[llvm-commits] Global Merge Pass for ARM

[Anton Korobeynikov]

Hello, Everyone

Please find the patch which can be viewed as some early approximation of
"section anchors" feature seen in gcc. 

It tries to solve the following problem: consider the code touches
several global variables at once, e.g.:

<=cut=>
static int foo[N], bar[N], baz[N];

for (i = 0; i < N; ++i) {
  foo[i] = bar[i] * baz[i];
}
<=cut=>

On ARM the addresses of 3 arrays should be kept in the registers, thus
this code has quite large register pressure (loop body):

        ldr     r1, [r5], #4
        ldr     r2, [r6], #4
        mul     r1, r2, r1
        str     r1, [r0], #4

Pass converts the code to something like:

<=cut=>
static struct {
  int foo[N];
  int bar[N];
  int baz[N];
} merged;

for (i = 0; i < N; ++i) {
  merged.foo[i] = merged.bar[i] * merged.baz[i];
}
<=cut=>

and in ARM code this becomes:
        ldr     r0, [r5, #40]
        ldr     r1, [r5, #80]
        mul     r0, r1, r0
        str     r0, [r5], #4

note that we saved 2 registers here.

This way only the address of the merged structured needs to be kept in
the registers. For the fields accesses ldr/str with offsets are used.
Pass correctly distinguishes constant and non-constant globals. Maximum
size of the struct dependes on the instruction set used (it's 4095 for
ARM/Thumb2 and 127 for Thumb1).

Maybe PPC can benefit from this pass as well, but I'm not yet sure.

Ok to commit?

-- 
With best regards, Anton Korobeynikov.

Faculty of Mathematics & Mechanics, Saint Petersburg State University.
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