Re: [PATCH take #2] PR middle-end/98865: Expand X*Y as X&-Y when Y is [0.1].

[Richard Biener <richard.guenther@gmail.com>]

On Wed, May 18, 2022 at 6:55 PM Roger Sayle <roger@nextmovesoftware.com> wrote:
>
>
> The patch is a revised solution for PR middle-end/98865 incorporating
> the feedback/suggestions from Richard Biener's review here:
> https://gcc.gnu.org/pipermail/gcc-patches/2022-May/593928.html
> Most significantly, this patch now performs the transformation/optimization
> during RTL expansion, where the target's rtx_costs can be used to determine
> whether the original multiplication (that may be possibly be implemented by
> a shift or lea) is cheaper than a negation and a bit-wise and.
>
> Previously the expression (x>>63)*y would be compiled with -O2 as
>         shrq    $63, %rdi
>         movq    %rdi, %rax
>         imulq   %rsi, %rax
>
> but with this patch now produces:
>         sarq    $63, %rdi
>         movq    %rdi, %rax
>         andq    %rsi, %rax
>
> Likewise the expression (x>>63)*135 [that appears in a hot-spot of the
> Botan AES-128 benchmark] was previously:
>
>         shrq    $63, %rdi
>         leaq    (%rdi,%rdi,8), %rdx
>         movq    %rdx, %rax
>         salq    $4, %rax
>         subq    %rdx, %rax
>
> now becomes:
>         movq    %rdi, %rax
>         sarq    $63, %rax
>         andl    $135, %eax
>
>
> 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.  Many thanks to Uros for the speedy review and approval of
> my x86_rtx_costs patch that enables this transformation on -m32 using the
> correct cost of DImode multiplication.  Ok for mainline?

Looks good to me

Thanks,
Richard.

> 2022-05-18  Roger Sayle  <roger@nextmovesoftware.com>
>
> gcc/ChangeLog
>         PR middle-end/98865
>         * expr.cc (expand_expr_real_2) [MULT_EXPR]:  Expand X*Y as X&Y
>         when both X and Y are [0, 1], X*Y as X&-Y when Y is [0,1] and
>         likewise X*Y as -X&Y when X is [0,1] using tree_nonzero_bits.
>
> gcc/testsuite/ChangeLog
>         PR middle-end/98865
>         * gcc.target/i386/pr98865.c: New test case.
>
>
> Thanks in advance,
> Roger
> --
>