[PATCH] ivopts: Improve code generated for very simple loops.

[Roger Sayle]

This patch tidies up the code that GCC generates for simple loops,
by selecting/generating a simpler loop bound expression in ivopts.
The original motivation came from looking at the following loop (from
gcc.target/i386/pr90178.c)

int *find_ptr (int* mem, int sz, int val)
{
  for (int i = 0; i < sz; i++)
    if (mem[i] == val)
      return &mem[i];
  return 0;
}

which GCC currently compiles to:

find_ptr:
        movq    %rdi, %rax
        testl   %esi, %esi
        jle     .L4
        leal    -1(%rsi), %ecx
        leaq    4(%rdi,%rcx,4), %rcx
        jmp     .L3
.L7:    addq    $4, %rax
        cmpq    %rcx, %rax
        je      .L4
.L3:    cmpl    %edx, (%rax)
        jne     .L7
        ret
.L4:    xorl    %eax, %eax
        ret

Notice the relatively complex leal/leaq instructions, that result
from ivopts using the following expression for the loop bound:
inv_expr 2:     ((unsigned long) ((unsigned int) sz_8(D) + 4294967295)
                * 4 + (unsigned long) mem_9(D)) + 4

which results from NITERS being (unsigned int) sz_8(D) + 4294967295,
i.e. (sz - 1), and the logic in cand_value_at determining the bound
as BASE + NITERS*STEP at the start of the final iteration and as
BASE + NITERS*STEP + STEP at the end of the final iteration.

Ideally, we'd like the middle-end optimizers to simplify
BASE + NITERS*STEP + STEP as BASE + (NITERS+1)*STEP, especially
when NITERS already has the form BOUND-1, but with type conversions
and possible overflow to worry about, the above "inv_expr 2" is the
best that can be done by fold (without additional context information).

This patch improves ivopts' cand_value_at by instead of using just
the tree expression for NITERS, passing the data structure that
explains how that expression was derived.  This allows us to peek
under the surface to check that NITERS+1 doesn't overflow, and in
this patch to use the SSA_NAME already holding the required value.

In the motivating loop above, inv_expr 2 now becomes:
(unsigned long) sz_8(D) * 4 + (unsigned long) mem_9(D)

And as a result, on x86_64 we now generate:

find_ptr:
        movq    %rdi, %rax
        testl   %esi, %esi
        jle     .L4
        movslq  %esi, %rsi
        leaq    (%rdi,%rsi,4), %rcx
        jmp     .L3
.L7:    addq    $4, %rax
        cmpq    %rcx, %rax
        je      .L4
.L3:    cmpl    %edx, (%rax)
        jne     .L7
        ret
.L4:    xorl    %eax, %eax
        ret


This improvement required one minor tweak to GCC's testsuite for
gcc.dg/wrapped-binop-simplify.c, where we again generate better
code, and therefore no longer find as many optimization opportunities
in later passes (vrp2).

Previously:

void v1 (unsigned long *in, unsigned long *out, unsigned int n)
{
  int i;
  for (i = 0; i < n; i++) {
    out[i] = in[i];
  }
}

on x86_64 generated:
v1:     testl   %edx, %edx
        je      .L1
        movl    %edx, %edx
        xorl    %eax, %eax
.L3:    movq    (%rdi,%rax,8), %rcx
        movq    %rcx, (%rsi,%rax,8)
        addq    $1, %rax
        cmpq    %rax, %rdx
        jne     .L3
.L1:    ret

and now instead generates:
v1:     testl   %edx, %edx
        je      .L1
        movl    %edx, %edx
        xorl    %eax, %eax
        leaq    0(,%rdx,8), %rcx
.L3:    movq    (%rdi,%rax), %rdx
        movq    %rdx, (%rsi,%rax)
        addq    $8, %rax
        cmpq    %rax, %rcx
        jne     .L3
.L1:    ret


This patch has been tested on x86_64-pc-linux-gnu with a make bootstrap
and make -k check with no new failures.  Ok for mainline?


2021-11-15  Roger Sayle  <roger@nextmovesoftware.com>

gcc/ChangeLog
        * tree-ssa-loop-ivopts.c (cand_value_at): Take a class
        tree_niter_desc* argument instead of just a tree for NITER.
        If we require the iv candidate value at the end of the final
        loop iteration, try using the original loop bound as the
        NITER for sufficiently simple loops.
        (may_eliminate_iv): Update (only) call to cand_value_at.

gcc/testsuite
        * gcc.dg/wrapped-binop-simplify.c: Update expected test result.


Thanks in advance,
Roger
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