mpir/doc/isa_abi_headache

Copyright 2000 Free Software Foundation, Inc.

This file is part of the GNU MP Library.

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Terms Used In This Document:
  ISA = Instruction Set Architecture.   The instructions the current
        processor provides.
  ABI = Application Binary Interface.  Specifies calling convention,
        type sizes, etc.
  AR64 = Arithmetic operations are 64-bit using 64-bit instructions
	 (E.g., addition, subtraction, load, store, of 64-bit integer types
	 are done with single instructions, not 32 bits at a time.)
  Environment = The operating system and compiler.

GMP is a very complex package to build since its speed is very
sensitive to the ISA and ABI.  For example, if the ISA provides 64-bit
instructions, it is crucial that GMP is configured to use them.

Most environments that run on a 64-bit ISA provide more than one ABI.
Typically one of the supported ABI's is a backward compatible 32-bit
ABI, and one ABI provides 64-bit addressing and `long' (sometimes
known as LP64).  But a few environments (IRIX, HP-UX) provide
intermediate ABI's using 32-bit addressing but allow efficient 64-bit
operations through a `long long' type.  For the latter to be useful to
GMP, the ABI must allow operations using the native 64-bit
instructions provided by the ISA, and allow passing of 64-bit
quantities atomically.

The ABI is typically chosen by means of command line options to the
compiler tools (gcc, cc, c89, nm, ar, ld, as).  Different environments
use different defaults, but as of this writing (May 2000) the
dominating default is to the plain 32-bit ABI in its most arcane form.

The GMP 3.0.x approach was to compile using the ABI that gives the
best performance.  That places the burden on users to pass special
options to the compiler when they compile their GMP applications.
That approach has its advantages and disadvantages.  The main
advantage is that users don't unknowingly get bad GMP performance.
The main disadvantage is that users' compiles (actually links) will
fail unless they pass special compiler options.

** SPARC

System vendors often confuse ABI, ISA, and implementation.  The worst
case is Solaris, were the unbundled compiler confuses ISA and ABI, and
the options have very confusing names.

     option		interpretation
     ======		==============
cc   -xarch=v8plus	ISA=sparcv9, ABI=V8plus (PTR=32, see below)
gcc  -mv8plus		ISA=sparcv9, ABI=V8plus (see below)
cc   -xarch=v9		ISA=sparcv9, ABI=V9 (implying AR=64, PTR=64)

It's hard to believe, but the option v8plus really means ISA=V9!

Solaris releases prior to version 7 running on a V9 CPU fails to
save/restore the upper 32 bits of the `i' and `l' registers.  The
`v8plus' option generates code that use as many V9 features as
possible under such circumstances.

** MIPS

The IRIX 6 compilers gets things right.  They have a clear
understanding of the differences between ABI and ISA.  The option
names are descriptive.

     option		interpretation
     ======		==============
cc   -n32		ABI=n32 (implying AR=64, PTR=32)
gcc  -mabi=n32		ABI=n32 (implying AR=64, PTR=32)
cc   -64		ABI=64 (implying AR=64, PTR=64)
gcc  -mabi=64		ABI=64 (implying AR=64, PTR=64)
cc   -mips3		ISA=mips3
gcc  -mips3		ISA=mips3
cc   -mips4		ISA=mips4
gcc  -mips4		ISA=mips4

** HP-PA

HP-UX is somewhat weird, but not as broken as Solaris.

     option		interpretation
     ======		==============
cc   +DA2.0		ABI=32bit (implying AR=64, PTR=32)
cc   +DD64		ABI=64bit (implying AR=64, PTR=64)

Code performing 64-bit arithmetic in the HP-UX 32-bit is not
compatible with the 64-bit ABI; the former has a calling convention
that passes/returns 64-bit integer quantities as two 32-bit chunks.

** PowerPC

While the PowerPC ABI's are capable of supporting 64-bit
registers/operations, the compilers under AIX are similar to Solaris'
cc in that they don't currently provide any 32-bit addressing with
64-bit arithmetic.

     option			interpretation
     ======			==============
cc   -q64			ABI=64bit (implying AR=64, PTR=64)
gcc  -maix64 -mpowerpc64	ABI=64bit (implying AR=64, PTR=64)