Re: [PATCH] RISC-V: zicond: remove bogus opt2 pattern

[Jeff Law via Gcc-patches]

On 8/30/23 15:57, Vineet Gupta wrote:

This was tripping up gcc.c-torture/execute/pr60003.c at -O1 since the
pattern semantics can't be expressed by zicond instructions.

This involves test code snippet:

       if (a == 0)
        return 0;
       else
        return x;
     }

which is equivalent to:  "x = (a != 0) ? x : a"

Isn't it

x = (a == 0) ? 0 : x

Which seems like it ought to fit zicond just fine.

If we take yours;

x = (a != 0) ? x : a

And simplify with the known value of a on the false arm we get:

x = (a != 0 ) ? x : 0;

Which is equivalent to

x = (a == 0) ? 0 : x;

So ISTM this does fit zicond just fine.

and matches define_insn "*czero.nez.<GPR:mode><X:mode>.opt2"

| (insn 41 20 38 3 (set (reg/v:DI 136 [ x ])
|        (if_then_else:DI (ne (reg/v:DI 134 [ a ])
|                (const_int 0 [0]))
|            (reg/v:DI 136 [ x ])
|            (reg/v:DI 134 [ a ]))) {*czero.nez.didi.opt2}

The corresponding asm pattern generates
     czero.nez x, x, a   ; %0, %2, %1
implying
     "x = (a != 0) ? 0 : a"

I get this from the RTL pattern:

x = (a != 0) ? x : a
x = (a != 0) ? x : 0

I think you got the arms reversed.

which is not what the pattern semantics are.

Essentially "(a != 0) ? x : a" cannot be expressed with CZERO.nez

Agreed, but I think you goof'd earlier :-)


Jeff