[match.pd PATCH] PR tree-optimization/114661: Generalize MULT_EXPR recognition.

[Roger Sayle]

This patch resolves PR tree-optimization/114661, by generalizing the set
of expressions that we canonicalize to multiplication.  This extends the
optimization(s) contributed (by me) back in July 2021.
https://gcc.gnu.org/pipermail/gcc-patches/2021-July/575999.html

The existing transformation folds (X*C1)^(X<<C2) into X*C3 when
allowed.  A subtlety is that for non-wrapping integer types, we
actually fold this into (int)((unsigned)X*C3) so that we don't
introduce an undefined overflow that wasn't in the original.
Unfortunately, this transformation confuses itself, as the type-safe
multiplication isn't recognized when further combining bit operations.
Fixed here by adding transforms to turn (int)((unsigned)X*C1)^(X<<C2)
into (int)((unsigned)X*C3) so that match.pd and EVRP can continue
to construct multiplications.

For the example given in the PR:

unsigned mul(unsigned char c) {
    if (c > 3) __builtin_unreachable();
    return c << 18 | c << 15 |
           c << 12 | c << 9 |
           c << 6 | c << 3 | c;
}

GCC on x86_64 with -O2 previously generated:

mul:    movzbl  %dil, %edi
        leal    (%rdi,%rdi,8), %edx
        leal    0(,%rdx,8), %eax
        movl    %edx, %ecx
        sall    $15, %edx
        orl     %edi, %eax
        sall    $9, %ecx
        orl     %ecx, %eax
        orl     %edx, %eax
        ret

with this patch we now generate:

mul:    movzbl  %dil, %eax
        imull   $299593, %eax, %eax
        ret

This patch has been tested on x86_64-pc-linux-gnu with make bootstrap
and make -k check, both with and without --target_board=unix{-m32}
with no new failures.  Ok for mainline?


2024-07-09  Roger Sayle  <roger@nextmovesoftware.com>

gcc/ChangeLog
        PR tree-optimization/114661
        * match.pd ((X*C1)|(X*C2) to X*(C1+C2)): Additionally recognize
        multiplications surrounded by casts to an unsigned type and back
        such as those generated by these transformations.

gcc/testsuite/ChangeLog
        PR tree-optimization/114661
        * gcc.dg/pr114661.c: New test case.


Thanks in advance,
Roger
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