In trying to implement a fast rotate with carry on the avr platform, it should
be possible to do a left shift on a register, and then add-with-carry that
register and __zero_reg__. i.e.:
asm volatile("lsl %0; adc %0, __zero_reg__" : "+r"(x) : "0"(x));
however, gcc doesn't seem to understand that the carry bit is set, and thus it
only emits the lsl instruction. The workaround is to split it into 2
statements:
asm volatile("lsl %0" : "+r"(x) : "0"(x));
asm volatile("adc %0, __zero_reg__" : "+r"(x) : "0"(x));
That generates the expected (correct) output.
--
Summary: GCC incorrectly optimizes inline assembly, resulting in
incorrect code
Product: gcc
Version: 4.3.4
Status: UNCONFIRMED
Severity: critical
Priority: P3
Component: target
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: dgrnbrg at mit dot edu
GCC build triplet: --target=avr --program-prefx=avr --enable-languages=c --
disable-
GCC host triplet: i688-linux
GCC target triplet: avr
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39628
