[Bug target/63635] New: Reduce toc relative address computation for multiple data access

[carrot at google dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63635

            Bug ID: 63635
           Summary: Reduce toc relative address computation for multiple
                    data access
           Product: gcc
           Version: 5.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: carrot at google dot com
            Target: powerpc64le

Currently ppc gcc generates two instructions to compute the address of non
local data. If the data layout is known to compiler, we can reduce one
instruction for the second and later data address computation. Following is an
example:

#include <stdio.h>

static int a,b,c;

void bar(int x)
{
  a = b = c = x;
}

int foo()
{
  return a+b+c;
}

int aa = 1;
int bb = 2;
int cc = 3;
int asdf()
{
  return aa + bb + cc; 
}

int main()
{
  printf("Hello");
  printf(", ");
  printf("world.\n");
}

Compile it with options -O2 -m64 -mvsx -mcpu=power8

Function asdf is compiled to:

asdf:
0:      addis 2,12,.TOC.-0b@ha
        addi 2,2,.TOC.-0b@l
        .localentry     asdf,.-asdf
        addis 3,2,.LANCHOR1@toc@ha       // A
        addis 10,2,.LANCHOR1+4@toc@ha    // B
        addis 9,2,.LANCHOR1+8@toc@ha     // C
        lwz 3,.LANCHOR1@toc@l(3)         // D
        lwz 10,.LANCHOR1+4@toc@l(10)     // E
        lwz 9,.LANCHOR1+8@toc@l(9)       // F
        add 3,3,10
        add 3,3,9
        extsw 3,3
        blr

...

        .globl cc
        .globl bb
        .globl aa
        .section        ".data"
        .align 2
        .set    .LANCHOR1,. + 0
        .type   aa, @object
        .size   aa, 4
aa:
        .long   1
        .type   bb, @object
        .size   bb, 4
bb:
        .long   2
        .type   cc, @object
        .size   cc, 4
cc:
        .long   3

Since the data layout of aa,bb,cc is known to compiler and their distance is
less than 64k, so the code sequence A-F can be optimized to:

        addis 3,2,.LANCHOR1@toc@ha
        addi  3,3,.LANCHOR1@toc@l
        lwz 10,4(3)
        lwz 9,8(3)
        lwz 3,0(3)

Other functions can be similarly optimized.