https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103799
Martin Liška <marxin at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Ever confirmed|0 |1
Last reconfirmed| |2021-12-22
Status|UNCONFIRMED |NEW
CC| |marxin at gcc dot gnu.org
--- Comment #2 from Martin Liška <marxin at gcc dot gnu.org> ---
(In reply to Andrew Pinski from comment #0)
> Take:
> auto f(char c)
> {
> int t1;
> switch (c)
> {
> case '1':
> t1 = 1;
> break;
> case '2':
> t1 = 2;
> break;
> case '3':
> t1 = 3;
> break;
> case '\0':
> t1 = 4;
> break;
> default:
> t1 = -1;
> }
> return t1;
> }
Confirmed. Here we have:
switch (_1) <default: <D.1986>, case 0: <D.1985>, case 49: <D.1982>, case 50:
<D.1983>, case 51: <D.1984>>
that's rejected by switch conversion [0, 51] range is quite sparse.
>
>
> auto f1(char c)
> {
> int t1;
> switch (c)
> {
> case '1':
> t1 = 1;
> break;
> case '2':
> t1 = 2;
> break;
> case '3':
> t1 = 3;
> break;
Here the first part is transformed by switch conversion as:
Linear transformation with A = 1 and B = -48
> default:
> if (c == '\0') t1 = 4; else t1 = -1;
And this is a classical
# t1_2 = PHI <4(4), -1(5), t1_5(3)>
So the idea of the transformation is basically to remove cases that "block"
and interesting switch optimization and handle these in default. Smart,
but not easy to achieve :P
> }
> return t1;
> }
>
> f1 produces better code than f.
