http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54571
Bug #: 54571
Summary: Missed optimization converting between bit sets
Classification: Unclassified
Product: gcc
Version: unknown
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: middle-end
AssignedTo: unassig...@gcc.gnu.org
ReportedBy: r...@gcc.gnu.org
When converting a bit set from one domain to another,code such as
if (old & OLD_X) new |= NEW_X;
if (old & OLD_Y) new |= NEW_Y;
is common. If OLD_X and NEW_X are single bits, then this conversion
need not include any conditional code. One can mask out OLD_X and
shift it left or right to become NEW_X. Or, vice versa, shift left
or right and then mask out NEW_X.
Indeed, it's probably preferable to perform the mask with the smaller
of OLD_X and NEW_X in order to maximize the possibility of having a
valid immediate operand to the logical insn.
A test case that would seem to cover the all the cases, including
converting logical not to bitwise not, would seem to be
int f1(int x, int y) { if (x & 1) y |= 1; return y; }
int f2(int x, int y) { if (x & 1) y |= 2; return y; }
int f3(int x, int y) { if (x & 2) y |= 1; return y; }
int g1(int x, int y) { if (!(x & 1)) y |= 1; return y; }
int g2(int x, int y) { if (!(x & 1)) y |= 2; return y; }
int g3(int x, int y) { if (!(x & 2)) y |= 1; return y; }
I'll also note that on the (presumably) preferred alternatives:
int h1(int x, int y) { return (x & 1) | y; }
int h2(int x, int y) { return ((x & 1) << 1) | y; }
int h3(int x, int y) { return ((x & 2) >> 1) | y; }
int k1(int x, int y) { return (~x & 1) | y; }
int k2(int x, int y) { return ((~x & 1) << 1) | y; }
int k3(int x, int y) { return ((~x >> 1) & 1) | y; }
there's some less-than-optimial code generated for k1 and k2.
