[Bug rtl-optimization/44281] [4.3/4.4/4.5/4.6 Regression] Global Register variable pessimisation

["adam at consulting dot net.nz" <gcc-bugzilla at gcc dot gnu dot org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=44281

--- Comment #8 from Adam Warner <adam at consulting dot net.nz> 2011-03-04 10:51:01 UTC ---
Jakub, I fail to see how your conclusion not to do this is supported by the
facts. There are:

(a) six global register variables (though the same effect can be observed with
one global register variable and -ffixed-rbx -ffixed-r12 -ffixed-r13
-ffixed-r14 -ffixed-r15)
(b) six function arguments
(c) one stack pointer

Therefore three registers remain free: %rax, %r10 and %r11. Only one free
register is required to generate the optimal code. GCC 4.5 can do this. GCC 4.6
can't.

The fact GCC outputs the assembly sequence "mov %rdi,%r10; mov %r10,%rdi" is
evidence of a bizarre cascade of bugs. Even rudimentary pinhole optimisation
could elide that assembly sequence.

Are you able to explain why GCC outputs assembly code for a register that is
never unmodified? %rdi remains unmodified. This has nothing to do with a
"compiler has much more limited choices in generating close to optimal
code". The compiler has the choice to use %rax, %r10 or %r11 to store the
address to jump to without spilling. There is no register pressure in this
example. One register is required. Three are available.