i386: Fix up <insn><dwi>3_doubleword_lowpart [PR112523]

i386: Fix up <insn><dwi>3_doubleword_lowpart [PR112523]

On Sun, Nov 12, 2023 at 09:03:42PM -0000, Roger Sayle wrote:
> This patch improves register pressure during reload, inspired by PR 97756.
> Normally, a double-word right-shift by a constant produces a double-word
> result, the highpart of which is dead when followed by a truncation.
> The dead code calculating the high part gets cleaned up post-reload, so
> the issue isn't normally visible, except for the increased register
> pressure during reload, sometimes leading to odd register assignments.
> Providing a post-reload splitter, which clobbers a single wordmode
> result register instead of a doubleword result register, helps (a bit).

Unfortunately this broke bootstrap on i686-linux, broke all ACATS tests
on x86_64-linux as well as miscompiled e.g. __floattisf in libgcc there
as well.

The bug is that shrd{l,q} instruction expects the low part of the input
to be the same register as the output, rather than the high part as the
patch implemented.
  split_double_mode (<DWI>mode, &operands[1], 1, &operands[1], &operands[3]);
sets operands[1] to the lo_half and operands[3] to the hi_half, so if
operands[0] is not the same register as operands[1] (rather than [3]) after
RA, we should during splitting move operands[1] into operands[0].

Your testcase:
> #define MASK60 ((1ul << 60) - 1)
> unsigned long foo (__uint128_t n)
> {
>   unsigned long a = n & MASK60;
>   unsigned long b = (n >> 60);
>   b = b & MASK60;
>   unsigned long c = (n >> 120);
>   return a+b+c;
> }

still has the same number of instructions.

Bootstrapped/regtested on x86_64-linux (where it e.g. turns
                === acats Summary ===
-# of unexpected failures       2328
+# of expected passes           2328
+# of unexpected failures       0
and fixes gcc.dg/torture/fp-int-convert-*timode.c FAILs as well)
and i686-linux (where it previously didn't bootstrap, but compared to
Friday evening's bootstrap the testresults are ok).

2023-11-14  Jakub Jelinek  <jakub@redhat.com>

	PR target/112523
	PR ada/112514
	* config/i386/i386.md (<insn><dwi>3_doubleword_lowpart): Move
	operands[1] aka low part of input rather than operands[3] aka high
	part of input to output if not the same register.