http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55181
Bug #: 55181
Summary: [4.7/4.8 Regression] Expensive shift loop where a
bit-testing instruction could be used
Classification: Unclassified
Product: gcc
Version: 4.7.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: other
AssignedTo: [email protected]
ReportedBy: [email protected]
Target: avr
The following C code:
unsigned char lfsr (unsigned long number)
{
unsigned char b = 0;
if (number & (1L << 29)) b++;
if (number & (1L << 13)) b++;
return b;
}
compiles to a right shift 29 bits of number which is very expensive because AVR
has no barrel shifter. Instead, a bit-testing instruction could be used which
takes just a few cycles and not more than 100 like with the right shift.
4.6.2 uses a bit testing instruction.
== Command line ==
