Re: [xstormy16 PATCH] Efficient HImode rotate left by a single bit.

[Jeff Law <jeffreyalaw@gmail.com>]

On 4/29/23 10:25, Roger Sayle wrote:
> This patch contains some minor tweak to xstormy16's machine description
> most significantly providing a pattern for HImode rotate left by a single
> bit that requires only two instructions.
> 
> unsigned short foo(unsigned short x)
> {
>    return (x << 1) | (x >> 15);
> }
> 
> currently with -O2 generates:
> foo:    mov r7,r2
>          shr r7,#15
>          shl r2,#1
>          or r2,r7
>          ret
> 
> with this patch, GCC now generates:
> foo:    shl r2,#1 | adc r2,#0
>          ret
> 
> Additionally neghi2 is converted to a define_insn (so that the RTL
> optimizers see the negation semantics), and HImode rotations by
> 8-bits can now be recognized and implemented using swpb.
> 
> This patch has been tested by building a cross-compiler to xstormy16-elf
> from x86_64-pc-linux-gnu and confirming the new test cases pass.
> Ok for mainline?
> 
> 
> 2023-04-29  Roger Sayle  <roger@nextmovesoftware.com>
> 
> gcc/ChangeLog
>          * config/stormy16/stormy16.md (neghi2): Convert from a define_expand
>          to a define_insn.
>          (*rotatehi_1): New define_insn for efficient 2 insn sequence.
>          (*rotatehi_8, *rotaterthi_8): New define_insn to emit a swpb.
> 
> gcc/testsuite/ChangeLog
>          * gcc.target/xstormy16/neghi2.c: New test case.
>          * gcc.target/rotatehi-1.c: Likewise.
It may be the case that exposing negation as a not + add sequence was 
thought to potentially produce better code by exposing the component 
instructions.  Or it may have simply been the case that nobody 
considered the tradeoffs.


Either way, I think the patch is fine.  As is always the case, figure 
~24hrs after committing we'll have test results.



jeff