+++ /dev/null
-
- X86 CPU FAMILY MPN SUBROUTINES
-
-
-This file has some notes on things common to all the x86 family code.
-
-
-
-ASM FILES
-
-The x86 .asm files are BSD style x86 assembler code, first put through m4
-for macro processing. The generic mpn/asm-defs.m4 is used, together with
-mpn/x86/x86-defs.m4. Detailed notes are in those files.
-
-The code is meant for use with GNU "gas" or a system "as". There's no
-support for assemblers that demand Intel style, and with gas freely
-available and easy to use that shouldn't be a problem.
-
-
-
-STACK FRAME
-
-m4 macros are used to define the parameters passed on the stack, and these
-act like comments on what the stack frame looks like too. For example,
-mpn_mul_1() has the following.
-
- defframe(PARAM_MULTIPLIER, 16)
- defframe(PARAM_SIZE, 12)
- defframe(PARAM_SRC, 8)
- defframe(PARAM_DST, 4)
-
-Here PARAM_MULTIPLIER gets defined as `FRAME+16(%esp)', and the others
-similarly. The return address is at offset 0, but there's not normally any
-need to access that.
-
-FRAME is redefined as necessary through the code so it's the number of bytes
-pushed on the stack, and hence the offsets in the parameter macros stay
-correct. At the start of a routine FRAME should be zero.
-
- deflit(`FRAME',0)
- ...
- deflit(`FRAME',4)
- ...
- deflit(`FRAME',8)
- ...
-
-Helper macros FRAME_pushl(), FRAME_popl(), FRAME_addl_esp() and
-FRAME_subl_esp() exist to adjust FRAME for the effect of those instructions,
-and can be used instead of explicit definitions if preferred.
-defframe_pushl() is a combination FRAME_pushl() and defframe().
-
-There's generally some slackness in redefining FRAME. If new values aren't
-going to get used, then the redefinitions are omitted to keep from
-cluttering up the code. This happens for instance at the end of a routine,
-where there might be just four register pops and then a ret, so FRAME isn't
-getting used.
-
-Local variables and saved registers can be similarly defined, with negative
-offsets representing stack space below the initial stack pointer. For
-example,
-
- defframe(SAVE_ESI, -4)
- defframe(SAVE_EDI, -8)
- defframe(VAR_COUNTER,-12)
-
- deflit(STACK_SPACE, 12)
-
-Here STACK_SPACE gets used in a "subl $STACK_SPACE, %esp" to allocate the
-space, and that instruction must be followed by a redefinition of FRAME
-(setting it equal to STACK_SPACE) to reflect the change in %esp.
-
-Definitions for pushed registers are only put in when they're going to be
-used. If registers are just saved and restored with pushes and pops then
-definitions aren't made.
-
-
-
-ASSEMBLER EXPRESSIONS
-
-Only addition and subtraction seem to be universally available, certainly
-that's all the Solaris 8 "as" seems to accept. If expressions are wanted
-then m4 eval() should be used.
-
-In particular note that a "/" anywhere in a line starts a comment in Solaris
-"as", and in some configurations of gas too.
-
- addl $32/2, %eax <-- wrong
-
- addl $eval(32/2), %eax <-- right
-
-Binutils gas/config/tc-i386.c has a choice between "/" being a comment
-anywhere in a line, or only at the start. FreeBSD patches 2.9.1 to select
-the latter, and as of 2.9.5 it's the default for GNU/Linux too.
-
-
-
-ASSEMBLER COMMENTS
-
-Solaris "as" doesn't support "#" commenting, using /* */ instead,
-unfortunately. For that reason "C" commenting is used (see asm-defs.m4) and
-the intermediate ".s" files have no comments.
-
-
-
-ZERO DISPLACEMENTS
-
-In a couple of places addressing modes like 0(%ebx) with a byte-sized zero
-displacement are wanted, rather than (%ebx) with no displacement. These are
-either for computed jumps or to get desirable code alignment. Explicit
-.byte sequences are used to ensure the assembler doesn't turn 0(%ebx) into
-(%ebx). The Zdisp() macro in x86-defs.m4 is used for this.
-
-Current gas 2.9.5 or recent 2.9.1 leave 0(%ebx) as written, but old gas
-1.92.3 changes it. In general changing would be the sort of "optimization"
-an assembler might perform, hence explicit ".byte"s are used where
-necessary.
-
-
-
-SHLD/SHRD INSTRUCTIONS
-
-The %cl count forms of double shift instructions like "shldl %cl,%eax,%ebx"
-must be written "shldl %eax,%ebx" for some assemblers. gas takes either,
-Solaris "as" doesn't allow %cl, gcc generates %cl for gas and NeXT (which is
-gas), and omits %cl elsewhere.
-
-For GMP an autoconf test is used to determine whether %cl should be used and
-the macros shldl, shrdl, shldw and shrdw in mpn/x86/x86-defs.m4 then pass
-through or omit %cl as necessary. See comments with those macros for usage.
-
-
-
-DIRECTION FLAG
-
-The x86 calling conventions say that the direction flag should be clear at
-function entry and exit. (See iBCS2 and SVR4 ABI books, references below.)
-
-Although this has been so since the year dot, it's not absolutely clear
-whether it's universally respected. Since it's better to be safe than
-sorry, gmp follows glibc and does a "cld" if it depends on the direction
-flag being clear. This happens only in a few places.
-
-
-
-POSITION INDEPENDENT CODE
-
-Defining the symbol PIC in m4 processing selects position independent code.
-This mainly affects computed jumps, and these are implemented in a
-self-contained fashion (without using the global offset table). The few
-calls from assembly code to global functions use the normal procedure
-linkage table.
-
-PIC is necessary for ELF shared libraries because they can be mapped into
-different processes at different virtual addresses. Text relocations in
-shared libraries are allowed, but that presumably means a page with such a
-relocation isn't shared. The use of the PLT for PIC adds a fixed cost to
-every function call, which is small but might be noticeable when working with
-small operands.
-
-Calls from one library function to another don't need to go through the PLT,
-since of course the call instruction uses a displacement, not an absolute
-address, and the relative locations of object files are known when libgmp.so
-is created. "ld -Bsymbolic" (or "gcc -Wl,-Bsymbolic") will resolve calls
-this way, so that there's no jump through the PLT, but of course leaving
-setups of the GOT address in %ebx that may be unnecessary.
-
-The %ebx setup could be avoided in assembly if a separate option controlled
-PIC for calls as opposed to computed jumps etc. But there's only ever
-likely to be a handful of calls out of assembler, and getting the same
-optimization for C intra-library calls would be more important. There seems
-no easy way to tell gcc that certain functions can be called non-PIC, and
-unfortunately many gmp functions use the global memory allocation variables,
-so they need the GOT anyway. Object files with no global data references
-and only intra-library calls could go into the library as non-PIC under
--Bsymbolic. Integrating this into libtool and automake is left as an
-exercise for the reader.
-
-
-
-SIMPLE LOOPS
-
-The overheads in setting up for an unrolled loop can mean that at small
-sizes a simple loop is faster. Making small sizes go fast is important,
-even if it adds a cycle or two to bigger sizes. To this end various
-routines choose between a simple loop and an unrolled loop according to
-operand size. The path to the simple loop, or to special case code for
-small sizes, is always as fast as possible.
-
-Adding a simple loop requires a conditional jump to choose between the
-simple and unrolled code. The size of a branch misprediction penalty
-affects whether a simple loop is worthwhile.
-
-The convention is for an m4 definition UNROLL_THRESHOLD to set the crossover
-point, with sizes < UNROLL_THRESHOLD using the simple loop, sizes >=
-UNROLL_THRESHOLD using the unrolled loop. If position independent code adds
-a couple of cycles to an unrolled loop setup, the threshold will vary with
-PIC or non-PIC. Something like the following is typical.
-
- ifdef(`PIC',`
- deflit(UNROLL_THRESHOLD, 10)
- ',`
- deflit(UNROLL_THRESHOLD, 8)
- ')
-
-There's no automated way to determine the threshold. Setting it to a small
-value and then to a big value makes it possible to measure the simple and
-unrolled loops each over a range of sizes, from which the crossover point
-can be determined. Alternately, just adjust the threshold up or down until
-there's no more speedups.
-
-
-
-UNROLLED LOOP CODING
-
-The x86 addressing modes allow a byte displacement of -128 to +127, making
-it possible to access 256 bytes, which is 64 limbs, without adjusting
-pointer registers within the loop. Dword sized displacements can be used
-too, but they increase code size, and unrolling to 64 ought to be enough.
-
-When unrolling to the full 64 limbs/loop, the limb at the top of the loop
-will have a displacement of -128, so pointers have to have a corresponding
-+128 added before entering the loop. When unrolling to 32 limbs/loop
-displacements 0 to 127 can be used with 0 at the top of the loop and no
-adjustment needed to the pointers.
-
-Where 64 limbs/loop is supported, the +128 adjustment is done only when 64
-limbs/loop is selected. Usually the gain in speed using 64 instead of 32 or
-16 is small, so support for 64 limbs/loop is generally only for comparison.
-
-
-
-COMPUTED JUMPS
-
-When working from least significant limb to most significant limb (most
-routines) the computed jump and pointer calculations in preparation for an
-unrolled loop are as follows.
-
- S = operand size in limbs
- N = number of limbs per loop (UNROLL_COUNT)
- L = log2 of unrolling (UNROLL_LOG2)
- M = mask for unrolling (UNROLL_MASK)
- C = code bytes per limb in the loop
- B = bytes per limb (4 for x86)
-
- computed jump (-S & M) * C + entrypoint
- subtract from pointers (-S & M) * B
- initial loop counter (S-1) >> L
- displacements 0 to B*(N-1)
-
-The loop counter is decremented at the end of each loop, and the looping
-stops when the decrement takes the counter to -1. The displacements are for
-the addressing accessing each limb, eg. a load with "movl disp(%ebx), %eax".
-
-Usually the multiply by "C" can be handled without an imul, using instead an
-leal, or a shift and subtract.
-
-When working from most significant to least significant limb (eg. mpn_lshift
-and mpn_copyd), the calculations change as follows.
-
- add to pointers (-S & M) * B
- displacements 0 to -B*(N-1)
-
-
-
-OLD GAS 1.92.3
-
-This version comes with FreeBSD 2.2.8 and has a couple of gremlins that
-affect gmp code.
-
-Firstly, an expression involving two forward references to labels comes out
-as zero. For example,
-
- addl $bar-foo, %eax
- foo:
- nop
- bar:
-
-This should lead to "addl $1, %eax", but it comes out as "addl $0, %eax".
-When only one forward reference is involved, it works correctly, as for
-example,
-
- foo:
- addl $bar-foo, %eax
- nop
- bar:
-
-Secondly, an expression involving two labels can't be used as the
-displacement for an leal. For example,
-
- foo:
- nop
- bar:
- leal bar-foo(%eax,%ebx,8), %ecx
-
-A slightly cryptic error is given, "Unimplemented segment type 0 in
-parse_operand". When only one label is used it's ok, and the label can be a
-forward reference too, as for example,
-
- leal foo(%eax,%ebx,8), %ecx
- nop
- foo:
-
-These problems only affect PIC computed jump calculations. The workarounds
-are just to do an leal without a displacement and then an addl, and to make
-sure the code is placed so that there's at most one forward reference in the
-addl.
-
-
-
-REFERENCES
-
-"Intel Architecture Software Developer's Manual", volumes 1 to 3, 1999,
-order numbers 243190, 243191 and 243192. Available on-line,
-
- ftp://download.intel.com/design/PentiumII/manuals/243190.htm
- ftp://download.intel.com/design/PentiumII/manuals/243191.htm
- ftp://download.intel.com/design/PentiumII/manuals/243192.htm
-
-"Intel386 Family Binary Compatibility Specification 2", Intel Corporation,
-published by McGraw-Hill, 1991, ISBN 0-07-031219-2.
-
-"System V Application Binary Interface", Unix System Laboratories Inc, 1992,
-published by Prentice Hall, ISBN 0-13-880410-9. And the "Intel386 Processor
-Supplement", AT&T, 1991, ISBN 0-13-877689-X. (These have details of ELF
-shared library PIC coding.)
-
-
-
-----------------
-Local variables:
-mode: text
-fill-column: 76
-End:
projects / ghc-hetmet.git / blobdiff