Re: [RFC PATCH v1 04/10] RISC-V: Support immediates in Zicond

[Jeff Law <jeffreyalaw@gmail.com>]

On 2/10/23 15:41, Philipp Tomsich wrote:
> When if-conversion encounters sequences using immediates, the
> sequences can't trivially map back onto czero.eqz/czero.nezt (even if
> benefitial) due to czero.eqz/czero.nez not having immediate forms.
> 
> This adds a splitter to rewrite opportunities for Zicond that operate
> on an immediate by first putting the immediate into a register to
> enable the non-immediate czero.eqz/czero.nez instructions to operate
> on the value.
> 
> Consider code, such as
> 
>    long func2 (long a, long c)
>    {
>      if (c)
>        a = 2;
>      else
>        a = 5;
>      return a;
>    }
> 
> which will be converted to
> 
>    func2:
> 	seqz    a0,a2
> 	neg     a0,a0
> 	andi    a0,a0,3
> 	addi    a0,a0,2
> 	ret
> 
> Following this change, we generate
> 
> 	li      a0,3
> 	czero.nez a0,a0,a2
> 	addi    a0,a0,2
> 	ret
> 
> This commit also introduces a simple unit test for if-conversion with
> immediate (literal) values as the sources for simple sets in the THEN
> and ELSE blocks. The test checks that the conditional-zero instruction
> (czero.eqz/nez) is emitted as part of the resulting branchless
> instruction sequence.
> 
> gcc/ChangeLog:
> 
> 	* config/riscv/zicond.md: Support immediates for
> 	czero.eqz/czero.nez through a splitter.
> 
> gcc/testsuite/ChangeLog:
> 
> 	* gcc.target/riscv/zicond-ifconv-imm.c: New test.
Same comment & resolution as with the #3 in this series.

A note though.  I've got Raphael looking at wiring this capability into 
the mov<mode>cc expander as well.  When complete that *may* make this 
patch largely obsolete.  But I don't think we necessarily need to wait 
for that to work to land this patch.

jeff