Hi Uros,
> From: Uros Bizjak <[email protected]>
> Sent: 28 December 2023 10:33
> On Fri, Dec 22, 2023 at 11:14 AM Roger Sayle <[email protected]>
> wrote:
> >
> > This patch resolves the failure of pr43644-2.c in the testsuite, a
> > code quality test I added back in July, that started failing as the
> > code GCC generates for 128-bit values (and their parameter passing)
> > has been in flux. After a few attempts at tweaking pattern
> > constraints in the hope of convincing reload to produce a more
> > aggressive (but potentially
> > unsafe) register allocation, I think the best solution is to use a
> > peephole2 to catch/clean-up this specific case.
> >
> > Specifically, the function:
> >
> > unsigned __int128 foo(unsigned __int128 x, unsigned long long y) {
> > return x+y;
> > }
> >
> > currently generates:
> >
> > foo: movq %rdx, %rcx
> > movq %rdi, %rax
> > movq %rsi, %rdx
> > addq %rcx, %rax
> > adcq $0, %rdx
> > ret
> >
> > and with this patch/peephole2 now generates:
> >
> > foo: movq %rdx, %rax
> > movq %rsi, %rdx
> > addq %rdi, %rax
> > adcq $0, %rdx
> > ret
> >
> > which I believe is optimal.
>
> How about simply moving the assignment to the MSB in the split pattern after
> the
> LSB calculation:
>
> [(set (match_dup 0) (match_dup 4))
> - (set (match_dup 5) (match_dup 2))
> (parallel [(set (reg:CCC FLAGS_REG)
> (compare:CCC
> (plus:DWIH (match_dup 0) (match_dup 1))
> (match_dup 0)))
> (set (match_dup 0)
> (plus:DWIH (match_dup 0) (match_dup 1)))])
> + (set (match_dup 5) (match_dup 2))
> (parallel [(set (match_dup 5)
> (plus:DWIH
> (plus:DWIH
>
> There is an earlyclobber on the output operand, so we are sure that
> assignments
> to (op 0) and (op 5) won't clobber anything.
> cprop_hardreg pass will then do the cleanup for us, resulting in:
>
> foo: movq %rdi, %rax
> addq %rdx, %rax
> movq %rsi, %rdx
> adcq $0, %rdx
>
> Uros.
I agree. This is a much better fix.
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?
2023-12-31 Uros Bizjak <[email protected]>
Roger Sayle <[email protected]>
gcc/ChangeLog
PR target/43644
* config/i386/i386.md (*add<dwi>3_doubleword_concat_zext): Tweak
order of instructions after split, to minimize number of moves.
gcc/testsuite/ChangeLog
PR target/43644
* gcc.target/i386/pr43644-2.c: Expect 2 movq instructions.
Thanks again (and Happy New Year).
Roger
--
diff --git a/gcc/config/i386/i386.md b/gcc/config/i386/i386.md
index e862368..6671274 100644
--- a/gcc/config/i386/i386.md
+++ b/gcc/config/i386/i386.md
@@ -6412,13 +6412,13 @@
"#"
"&& reload_completed"
[(set (match_dup 0) (match_dup 4))
- (set (match_dup 5) (match_dup 2))
(parallel [(set (reg:CCC FLAGS_REG)
(compare:CCC
(plus:DWIH (match_dup 0) (match_dup 1))
(match_dup 0)))
(set (match_dup 0)
(plus:DWIH (match_dup 0) (match_dup 1)))])
+ (set (match_dup 5) (match_dup 2))
(parallel [(set (match_dup 5)
(plus:DWIH
(plus:DWIH
diff --git a/gcc/testsuite/gcc.target/i386/pr43644-2.c
b/gcc/testsuite/gcc.target/i386/pr43644-2.c
index d470b0a..3316ac6 100644
--- a/gcc/testsuite/gcc.target/i386/pr43644-2.c
+++ b/gcc/testsuite/gcc.target/i386/pr43644-2.c
@@ -6,4 +6,4 @@ unsigned __int128 foo(unsigned __int128 x, unsigned long long y)
return x+y;
}
-/* { dg-final { scan-assembler-times "movq" 1 } } */
+/* { dg-final { scan-assembler-times "movq" 2 } } */
