Re: [PATCH] RISC-V: bitmanip: use bexti for "(a & (1 << BIT_NO)) ? 0 : -1"

[Jeff Law <jeffreyalaw@gmail.com>]

On 11/8/22 12:55, Philipp Tomsich wrote:
> Consider creating a polarity-reversed mask from a set-bit (i.e., if
> the bit is set, produce all-ones; otherwise: all-zeros).  Using Zbb,
> this can be expressed as bexti, followed by an addi of minus-one.  To
> enable the combiner to discover this opportunity, we need to split the
> canonical expression for "(a & (1 << BIT_NO)) ? 0 : -1" into a form
> combinable into bexti.
>
> Consider the function:
>      long f(long a)
>      {
>        return (a & (1 << BIT_NO)) ? 0 : -1;
>      }
> This produces the following sequence prior to this change:
> 	andi	a0,a0,16
> 	seqz	a0,a0
> 	neg	a0,a0
> 	ret
> Following this change, it results in:
> 	bexti	a0,a0,4
> 	addi	a0,a0,-1
> 	ret
>
> gcc/ChangeLog:
>
> 	* config/riscv/bitmanip.md: Add a splitter to generate
>            polarity-reversed masks from a set bit using bexti + addi.
>
> gcc/testsuite/ChangeLog:
>
> 	* gcc.target/riscv/zbs-bexti.c: New test.

OK


Jeff