Re: [PATCH 2/5] [RISC-V] Generate Zicond instruction for basic semantics

[Jeff Law <jeffreyalaw@gmail.com>]

On 7/19/23 04:11, Xiao Zeng wrote:
> This patch completes the recognition of the basic semantics
> defined in the spec, namely:
> 
> Conditional zero, if condition is equal to zero
>    rd = (rs2 == 0) ? 0 : rs1
> Conditional zero, if condition is non zero
>    rd = (rs2 != 0) ? 0 : rs1
> 
> gcc/ChangeLog:
> 
> 	* config/riscv/riscv.md: Include zicond.md
> 	* config/riscv/zicond.md: New file.
> 
> gcc/testsuite/ChangeLog:
> 
> 	* gcc.target/riscv/zicond-primitiveSemantics.c: New test.
So I played with this a bit today.  I originally thought that using 
match_dup was the right way to go for those 4 secondary patterns.  But 
after further pondering it's not ideal.

match_dup will require pointer equality within the RTL structure.  That 
could inhibit detection in two cases.  First, SUBREGs.   SUBREGs are not 
shared.  So we'd never match if we had a SUBREG expression.

Second, post register allocation we can have the same looking RTX, but 
it may not be pointer equal.

The SUBREG issue also means that we don't want to use a REGNO (x) == 
REGNO (y) style check because those macros are only valid on REG 
expressions.  We could strip the SUBREG, but that's usually awkward to 
do in a pattern's condition.

The net result is we probably should use rtx_equal_p which I was hoping 
to avoid.  I'm testing with that change to the 4 secondary patterns 
right now.  Assuming that passes (and I have no reason to think it 
won't) then I'll go ahead and commit #1 and #2 from this series which is 
all I have time for today.



Jeff