Compiling x264 for iOS 7

问题:

I'm getting error on compiling x264 for iOS.

I have Xcode Version 5.0 (5A1413) with Apple LLVM version 5.0 (clang-500.2.75) (based on LLVM 3.3svn). I'm compiling x264-snapshot-20130925-2245.

Config:

CC=/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/clang ./configure \
--host=arm-apple-darwin \
--sysroot=/Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS7.0.sdk \
--prefix=armv7 \
--extra-cflags='-arch armv7' \
--extra-ldflags="-L/Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS7.0.sdk/usr/lib/system -arch armv7" \
--enable-pic \
--enable-static

Getting error:

common/arm/cpu-a.S:29:7: error: unknown token in expression
.align
      ^
common/arm/cpu-a.S:139:5: error: instruction 'suble' can not set flags, but 's' suffix specified
    subles ip, ip, #1
    ^

回答1:

The relevant change in the Xcode 5 toolchain is that the LLVM compiler now defaults to using the built-in assembler, and the built-in assembler requires more strict adherence to the ARM Unified Assembly Language.

There are two ways to get it to compile with the Xcode 5 toolchain:

1. Give clang the flag -no-integrated-as. Adding it to --extra-cflags ought to work. (The flag worked for me compiling individual files but I never worked it into configure.) Consider this a workaround.

2. Fix the assembly source code in x264's common/arm subdirectory. This is pretty easy actually, and it's what I did. This is the right fix. BTW, I'm about to submit a patch to x264 with these changes.

The assembler emits many errors and they fall into four categories:

• In cpu-a.S, the ".align" directive should be ".align 2". (Apparently it used to default to 2, now the 2 must be explicit.)

• Several subles and sublts instructions in multiple files. These are variants of "sub" (subtract), followed by a condition (2 characters) and the "s" suffix. Now the "s" has to precede the condition. Thus "subles" => "subsle" and "sublts" => "subslt".

• A fair number of ldrd instructions in various files. This instruction means "load register, double (from memory)". It loads 2 32-bit words from memory into registers. It used to be OK to name only the first register; now both need to be named. They're always adjacent. So "ldrd r2, whatever" needs to become "ldrd r2, r3, whatever". "ldrd r6, something" becomes "ldrd r6, r7, something". Etc.

• In pixel-a.S, there's an instruction "vmov.32 r0, r1, d0". This is incorrect. vmov.32 means move a 32-bit quantity, yet the arguments say to move d0 (64 bits) into r0 and r1. Apparently the old compiler took the ".32" part as a hint. I believe it should be "vmov r0, r1, d0" and that change works for me - but I don't have absolute proof that is the correct instruction.

Many thanks to gparker on the Apple Developer Forum! I could not have figured this out without his/her help. Link to forum discussion, Apple ID required.

回答2:

So far the only solution seems to be --disable-asm.

回答3:

I think disabling assembler optimizations is a bad solution.

After a long research I've found the root of the problem: clang during assembler compilation uses ASFLAGS not CFLAGS, so adding --extra-asflags="-arch armv7" solves the problem

./configure \
--host=arm-apple-darwin \
--sysroot=/Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS.sdk \
--prefix=armv7 \
--extra-cflags="-arch armv7" \
--extra-asflags="-arch armv7" \
--extra-ldflags="-arch armv7" \
--enable-pic \
--enable-static

NOTE: For bitcode support just add -fembed-bitcode to all extra flags parameters