https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61839
Richard Biener <rguenth at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Keywords| |missed-optimization
Status|UNCONFIRMED |NEW
Last reconfirmed| |2014-07-23
Summary|More optimize opportunity |More optimize opportunity
| |for VRP
Ever confirmed|0 |1
Known to fail| |4.10.0
--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
Confirmed. This is straight-line (A) code vs. if-code (B) that makes the
difference, introduced by fold already:
--- a/t.c.003t.original 2014-07-23 14:48:48.984335370 +0200
+++ b/t.c.003t.original 2014-07-23 14:46:19.224345681 +0200
@@ -11,7 +11,7 @@
int a = -1;
volatile unsigned int b = 1;
int c = 1;
- c = b != 0 ? 486097858 : 972195717;
+ c = a + 972195718 >> (b != 0);
if (c == 486097858)
{
(void) 0;
in the bad case we have
<bb 2>:
b ={v} 1;
b.0_3 ={v} b;
_4 = b.0_3 != 0;
_5 = (int) _4;
c_6 = 972195717 >> _5;
if (c_6 == 486097858)
...
until the very end, not transforming c_6. Note that VRP could do the
missing transform as it knows that _5 is [0, 1] (it has to jump through
the shift - the value-range for the shift itself is too broad).
If written this kind of transform should be applied more generally, not
just for shifts. It basically wants to ask whether a conditional test
can be carried out against another SSA name (and another constant) if
an intermediate compute can be omitted in that case.
