10:45 Sun, 16 Dec 2012 PST -0800

FatELF Toolchain Drivers

Introduction

One of the handy features in Mac OS X's universal binary support is the ability to easily compile a binary for multiple architectures with one command. This simplifies the process of producing a binary for multiple devices, avoiding the need to run the build multiple times and manually stitch together the results.

For example, one could build a mixed armv7, armv7s, and armv6 binary -- with thumb enabled only for armv6 -- with the following command:

gcc -arch armv7 -arch armv7s -arch armv6 -Xarch_armv6 -mthumb

In reality, the compiler is still being run multiple times; an arch-aware driver handles automatically running the compilers, writing the build results to temporary files, and then stitching them together with lipo(1). In the case of clang, the code to do this actually lives in clang itself. However, in the case of gcc, Apple implemented a front-end driver that sits in front of the actual gcc executables, called driverdriver.c. A similar driver is also supplied for as(1); it handles calling out to the correct assembler, but only supports a single -arch flag.

Haiku Implementation

In implementing FatELF on Haiku, I needed the functionality provided by these drivers. Unfortunately, they were written somewhat non-portably for Mac OS X, and in the case of the gcc driverdriver.c, relied on internal GCC APIs that have been removed in later versions of GCC.

To solve this, I went ahead and wrote new front-end drivers that work with modern gcc(1) and as(1) versions. I implemented them as part of the Haiku fatelf-tools project, but they're meant to be portable to multiple hosts, and should be straight-forward to hoist out for other purposes. These could be used, for example, to support building universal binaries using more recent gcc releases on Mac OS X, where the traditional driverdriver.c cannot otherwise be used (due to reliance on GCC APIs that have now been removed).

If you want to review the initial implementation, the drivers can be found in my haiku-buildtools repository: fatelf-gcc.c, fatelf-as.c.

With these tools, I can build multi-arch Haiku FatELF binaries using the standard GCC toolchain:

landonf@lambda:fatelf> ./fatelf-gcc -arch i386 -arch x86_64 -c foo.c -o foo.o                   
landonf@lambda:fatelf> ./fatelf-info foo.o
foo.o: FatELF format version 1
2 records.
Binary at index #0:
  OSABI 0 (sysv: UNIX System V) version 0,
  32 bits
  Littleendian byteorder
  Machine 3 (i386: Intel 80386)
  Offset 4096
  Size 547
  Target name: 'i386:32bits:le:sysv:osabiver0' or 'record0'
Binary at index #1:
  OSABI 0 (sysv: UNIX System V) version 0,
  64 bits
  Littleendian byteorder
  Machine 62 (x86_64: AMD x86-64)
  Offset 8192
  Size 799
  Target name: 'x86_64:64bits:le:sysv:osabiver0' or 'record1'

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