On Fri, Dec 10, 2021 at 12:58 PM Roger Sayle <ro...@nextmovesoftware.com> wrote:
>
>
> While working on a middle-end patch to more aggressively use highpart
> multiplications on targets that support them, I noticed that the RTL
> expanded by the x86 backend interacts poorly with register allocation
> leading to suboptimal code.
>
> For the testcase,
> typedef int __attribute ((mode(TI))) ti_t;
> long foo(long x)
> {
> return ((ti_t)x * 19065) >> 64;
> }
>
> we'd like to avoid:
> foo: movq %rdi, %rax
> movl $19065, %edx
> imulq %rdx
> movq %rdx, %rax
> ret
>
> and would prefer:
> foo: movl $19065, %eax
> imulq %rdi
> movq %rdx, %rax
> ret
>
> This patch provides a pair of peephole2 transformations to tweak the
> spills generated by reload, and at the same time replaces the current
> define_expand with define_insn patterns using the new [su]mul_highpart
> RTX codes. I've left the old-style patterns in the machine description
> for the time being, but plan to remove these once my planned middle-end
> improvements make them obsolete.
>
> This patch has been tested on x86_64-pc-linux-gnu (both with and without
> the middle-end changes) by make bootstrap and make -k check with no new
> failures. The new test case, which currently passes, ensures that the
> code we generate isn't adversely affected by changes outside the backend.
> Ok for mainline?
>
>
> 2021-12-10 Roger Sayle <ro...@nextmovesoftware.com>
>
> gcc/ChangeLog
> * config/i386/i386.md (any_mul_highpart): New code iterator.
> (sgnprefix, s): Add attribute support for [su]mul_highpart.
> (<s>mul<mode>3_highpart): Delete expander.
> (<s>muldi3_highpart, <s>mulsi3_highpart, <s>mulsi32_highpart_zext):
> New define_insn patterns.
> (define_peephole2): Tweak the register allocation for the above
> instructions after reload.
>
> gcc/testsuite/ChangeLog
> * gcc.target/i386/smuldi3_highpart.c: New test case.
Can you please merge <s>muldi3_highpart and *<s>mulsi3_highpart using
SWI48 mode iterator?
Uros.
>
>
> Thanks in advance,
> Roger
> --
>
