https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71343
Richard Biener <rguenth at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|UNCONFIRMED |NEW
Last reconfirmed| |2016-05-30
Component|rtl-optimization |tree-optimization
Ever confirmed|0 |1
--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
Confirmed and eventually not helped by factoring *4 early:
return (a << 2) + (b << 2) == (a + b) * 4;
adjusted testcase for that:
unsigned int test12(unsigned int a , unsigned int b)
{
unsigned int tem1 = a * 4;
unsigned int tem2 = b * 4;
return ((a << 2) + (b << 2)) == tem1 + tem2;
}
also fails to optimize. In the end this is a missed canonicalization though
semantically shifts are not always equal to multiplication (they are for
unsigned arithmetic).
CSE could also handle this (it also "fails" to handle x + x as x * 2, but we
canonicalize that now).
I'd say the multiplication is canonical here.
We do optimize
unsigned int test12(unsigned int a , unsigned int b)
{
unsigned int tem1 = a / 4;
unsigned int tem2 = b / 4;
return ((a >> 2) + (b >> 2)) == tem1 + tem2;
}
but pretty late in DOM2. VRP1 is replacing the division by a shift.
