On 8/31/23 06:51, Jeff Law wrote:
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.
Logically they are equivalent, but ....
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.
I could very well be mistaken, but define_insn is a pattern match and
opt2 has *ne* so the expression has to be in != form and thus needs to
work with that condition. No ?
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
This is the issue, for ne, czero.nez can only express
x = (a != 0) ? 0 : x
I think you got the arms reversed.
What I meant was czero.nez as specified in RTL pattern would generate x
= (a != 0) ? 0 : a, whereas pattern's desired semantics is (a != 0) ? x : 0
And that is a problem because after all equivalents/simplifications, a
ternary operation's middle operand has to be zero to map to czero*, but
it doesn't for the opt2 RTL semantics.
I've sat on this for 2 days, trying to convince myself I was wrong, but
as it stands, it was generating wrong code in the test which is fixed
after the patch.
Thx,
-Vineet
