On 06/10/10 14:44, Bernd Schmidt wrote:
On 06/10/2010 10:37 PM, Jeff Law wrote:
Compile the attached with -O2 on x86-unknown-linux-gnu and review the
.ira dump for main()
starting the processing of deferred insns
ending the processing of deferred insns
df_analyze called
Building IRA IR
starting the processing of deferred insns
ending the processing of deferred insns
df_analyze called
init_insns for 59: (insn_list:REG_DEP_TRUE 5 (nil))
Reg 59 has equivalence, initial gains 4000
[...]
r59: preferred NO_REGS, alternative NO_REGS, cover NO_REGS
[...]
Disposition:
0:r59 l0 mem
Ultimately I think reload is cleaning this up, but it seems awful
strange to have a pseudo/allocno which clearly should be allocated to a
hard GPR preferring NO_REGS and from an allocation standpoint living in
memory.
From the above, I don't see the problem. Reg 59 is detected as
reg_equiv_invariant, which means if we don't allocate a hard reg to it,
we can substitute the invariant everywhere and save the initializing
instruction. As far as I can tell this is working exactly as intended.
But shouldn't having an invariant form just decrease the priority for
pseudo 59 to get a hard register? The NO_REGS preferencing will totally
disable register allocation for pseudo 59.
Just because we have an invariant equivalent form doesn't mean we should
totally disable allocation -- if a hard reg is available, we're
sometimes going to be better off using a hard reg to hold the value
rather than substituting the equivalent form at each use of the pseudo.
Jeff
Bernd
