On 5/19/2026 12:14 AM, Richard Biener wrote:
On Mon, May 18, 2026 at 4:37 PM Raphael Zinsly
<[email protected]> wrote:
On Fri, May 15, 2026 at 10:09 AM Richard Biener <[email protected]> wrote:
On Wed, 13 May 2026, Raphael Moreira Zinsly wrote:
...
+/* Simplify (a < 0) ? ARG1 OP CONST : ARG1 to ((a < 0) ? 0 : CONST) OP ARG1
+ when CONST is a 2^n-1 constant. */
+(for op (plus bit_ior bit_xor)
+ (simplify
+ (cond (lt @0 integer_zerop@1) (op:c @2 INTEGER_CST@3) @2)
No :c necessary on 'op'
Ok.
+ (if (wi::exact_log2 (wi::to_wide (@3) + 1) != -1)
Why is it cheap to splat the sign bit? If it's cheap to splat
the sign bit it's cheap to select between zero and a constant:
<splat-sign-bit-to-full-width> & COSTANT.
That you leave in the (cond ...) hints strongly in that it's
an oddly special "canonicalization" and that it is really
a target dependent RTL thing?
It is a canonicalization, but I believe it's worth it.
The message of this commit is misleading, which is my mistake, I was so
immersed in trying to generate a splat sign bit that I kept the original commit
description without much thought.
I'm in fact generating a conditional move that can be transformed into a splat
sign bit in the targets that benefit from it.
I will describe it better in V2.
The current code produces for 'c < 0 ? a : a + 7':
if (c_2(D) >= 0)
goto <bb 3>;
else
goto <bb 4>;
<bb 3>
_4 = a_3(D) + 7;
<bb 4>
# _1 = PHI <a_3(D)(2), _4(3)>
return _1;
After this patch we get:
_1 = c_2(D) < 0;
_4 = _1 ? 0 : 7;
_8 = a_3(D) + _4;
return _8;
Is it reasonable?
I think so. But as said, why only for 7 and not for 42?
2^n-1 is interesting because there's a highly generic way to synthesize
that sequence. It's just two shifts. So long as you have a barrel
shifter, that's going to be pretty efficient. Arbitrary constants aren't
as clear cut as you have to worry about whether or not the nonzero
constant can be encoded by the target.
Granted we're in gimple, so one could make an argument that these are
low level issues that aren't applicable.
The way I've generally encouraged folks to think about these
transformations is to look at both gimple and RTL simplification. I
think in this case we were initially looking at the RTL side of things,
then concluded the idea was sound for gimple without too much
consideration for whether or not it ought to be generalized to more
constants in gimple.
jeff
