[PATCH PR57534]Support strength reduction for MEM_REF in slur

[bin.cheng]

Hi,
As noted in PR57534 comment #33, SLSR currently doesn't strength reduce memory
references in reported cases, which conflicts with its comment at the beginning of file.
The main reason is in functions slsr_process_ref and restructure_reference which
rejects MEM_REF by handled_compoenent_p in the first place.  This patch identifies
and creates CAND_REF for MEM_REF by restructuring base/offset.

Note the patch only affects cases with multiple reducible MEM_REF.

Also note, with this patch, [base + cst_offset] addressing mode would be generally
preferred.  I need to adjust three existing tests:
* gcc.dg/tree-ssa/isolate-5.c
* gcc.dg/tree-ssa/ssa-hoist-4.c
Though address computation is reduced out of memory reference, the generated 
assembly is not worsened.

* gcc.dg/tree-ssa/slsr-3.c
The generated assembly has two more instructions:
<       movslq  %edx, %rcx
<       movl    (%rsi,%rcx,4), %r9d
<       leaq    0(,%rcx,4), %rax
<       leal    2(%r9), %r8d
<       movl    %r8d, (%rdi,%rcx,4)
<       movl    4(%rsi,%rax), %ecx
<       addl    $2, %ecx
<       movl    %ecx, 4(%rdi,%rax)
<       movl    8(%rsi,%rax), %ecx
<       addl    $2, %ecx
<       movl    %ecx, 8(%rdi,%rax)
<       movl    12(%rsi,%rax), %ecx
<       addl    $2, %ecx
<       movl    %ecx, 12(%rdi,%rax)
---
>       movslq  %edx, %rax
>       salq    $2, %rax
>       addq    %rax, %rsi
>       addq    %rax, %rdi
>       movl    (%rsi), %eax
>       addl    $2, %eax
>       movl    %eax, (%rdi)
>       movl    4(%rsi), %eax
>       addl    $2, %eax
>       movl    %eax, 4(%rdi)
>       movl    8(%rsi), %eax
>       addl    $2, %eax
>       movl    %eax, 8(%rdi)
>       movl    12(%rsi), %eax
>       addl    $2, %eax
>       movl    %eax, 12(%rdi)

Seems to me this is not deteriorating and "salq" can be saved by two forward propagation.

Bootstrap and test on x86_64, any comments?

Thanks,
bin

2019-05-15  Bin Cheng  <bin.cheng@linux.alibaba.com>

        PR tree-optimization/57534
        * gimple-ssa-strength-reduction.c (restructure_base_offset): New.
        (restructure_reference): Call restructure_base_offset when offset is
        NULL.
        (slsr_process_ref): Handle MEM_REF.

2018-05-15  Bin Cheng  <bin.cheng@linux.alibaba.com>

        PR tree-optimization/57534
        * gcc.dg/tree-ssa/pr57534.c: New test.
        * gcc.dg/tree-ssa/isolate-5.c: Adjust checking strings.
        * gcc.dg/tree-ssa/slsr-3.c: Ditto.
        * gcc.dg/tree-ssa/ssa-hoist-4.c: Ditto.
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