Re: Aw: Re: C3600 kernel/64bit 4.* slow IO due to -mlong-calls
On 2018-03-16 7:25 AM, Helge Deller wrote:
kernel gcc binutils with mlong without mlong
4.15.7 4.9.3 2.25.1 13.4 MB/s 27.0 MB/s
4.15.7 6.4.0 2.25.1 13.4 MB/s 27.0 MB/s
4.15.7 6.4.0 2.29.1 14.4 MB/s 25.0 MB/s
Interesting bad results!
It's hard to understand why the performance would deteriorate so much
but I see essentially the same behavior.
Speaking of debian kernel, it's nearly impossible to link a kernel without mlong-calls.
Compiling without mlong-calls generates this (R_PARISC_PCREL22F):
b,l external_func,%r2
nop
On PA 2.0, this is a 22 bit pc-relative call that has a branch distance
of 8 MB. We have no stub support
in the gnu 64-bit linker. If we had stub support, this would be best
solution.
In addition to the argument registers, the argument pointer needs to be
loaded for each call.
With -mlong-calls it is much more complex:
.LC0:
.dword P%external_func
.globl a
a:
addil LT'.LC0,%r27
ldd RT'.LC0(%r1),%r28
ldd 0(%r28),%r28
ldd 16(%r28),%r2
bve,l (%r2),%r2
This is standard 64-bit indirect call. It calls via a function
descriptor. It assumes the PIC register may change
and the callee may be in a different space (i.e., 64-bit hpux runtime).
The bve instruction is specific to PA 2.0.
b
In the kernel, we probably don't need the load of the new PIC register
(omitted from the above).
Since our kernel is running in the first 4GB of RAM (even on 64bit), couldn't we instead
introduce a gcc option, e.g. "-mkernel-indirect-calls", which translates to:
ldil L%external_func, %r2 // R_PARISC_DIR21L
ldo R%external_func(%r2), %r2 // R_PARISC_DIR14R
bve,l (%r2),%r2
Another option is to use ble (i.e., call sequence generated using
-mfast-indirect-calls). It yields the same length
call sequence as your above sequence and it works on both PA 1.x and 2.0.
The above sequence is not PIC. What about modules?
In the above three sequences, there is a delay slot after the branch
which might be filled by the compiler with a
useful instruction.
Does -mfast-indirect-calls has any effect at all?
I haven't seen any difference when using this option.
At the moment, this option only applies to the 32-bit compiler.
Thoughts?
I don't remember any huge increase in gcc build time with -mlong-calls.
Calls don't usually dominate performance.
Dave
--
John David Anglin dave.anglin@bell.net
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