libgcrypt-1.4.6/mpi/pentium4/README

Copyright 2001 Free Software Foundation, Inc.

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                   INTEL PENTIUM-4 MPN SUBROUTINES


This directory contains mpn functions optimized for Intel Pentium-4.

The mmx subdirectory has routines using MMX instructions, the sse2
subdirectory has routines using SSE2 instructions.  All P4s have these, the
separate directories are just so configure can omit that code if the
assembler doesn't support it.


STATUS

                                cycles/limb

	mpn_add_n/sub_n            4 normal, 6 in-place

	mpn_mul_1                  4 normal, 6 in-place
	mpn_addmul_1               6
	mpn_submul_1               7

	mpn_mul_basecase           6 cycles/crossproduct (approx)

	mpn_sqr_basecase           3.5 cycles/crossproduct (approx)
                                   or 7.0 cycles/triangleproduct (approx)

	mpn_l/rshift               1.75



The shifts ought to be able to go at 1.5 c/l, but not much effort has been
applied to them yet.

In-place operations, and all addmul, submul, mul_basecase and sqr_basecase
calls, suffer from pipeline anomalies associated with write combining and
movd reads and writes to the same or nearby locations.  The movq
instructions do not trigger the same hardware problems.  Unfortunately,
using movq and splitting/combining seems to require too many extra
instructions to help.  Perhaps future chip steppings will be better.



NOTES

The Pentium-4 pipeline "Netburst", provides for quite a number of surprises.
Many traditional x86 instructions run very slowly, requiring use of
alterative instructions for acceptable performance.

adcl and sbbl are quite slow at 8 cycles for reg->reg.  paddq of 32-bits
within a 64-bit mmx register seems better, though the combination
paddq/psrlq when propagating a carry is still a 4 cycle latency.

incl and decl should be avoided, instead use add $1 and sub $1.  Apparently
the carry flag is not separately renamed, so incl and decl depend on all
previous flags-setting instructions.

shll and shrl have a 4 cycle latency, or 8 times the latency of the fastest
integer instructions (addl, subl, orl, andl, and some more).  shldl and
shrdl seem to have 13 and 15 cycles latency, respectively.  Bizarre.

movq mmx -> mmx does have 6 cycle latency, as noted in the documentation.
pxor/por or similar combination at 2 cycles latency can be used instead.
The movq however executes in the float unit, thereby saving MMX execution
resources.  With the right juggling, data moves shouldn't be on a dependent
chain.

L1 is write-through, but the write-combining sounds like it does enough to
not require explicit destination prefetching.

xmm registers so far haven't found a use, but not much effort has been
expended.  A configure test for whether the operating system knows
fxsave/fxrestor will be needed if they're used.



REFERENCES

Intel Pentium-4 processor manuals,

	http://developer.intel.com/design/pentium4/manuals

"Intel Pentium 4 Processor Optimization Reference Manual", Intel, 2001,
order number 248966.  Available on-line:

	http://developer.intel.com/design/pentium4/manuals/248966.htm



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