Wednesday, May 21, 2014

Why objc_autoreleaseReturnValue Differs for x86_64 and ARM


The implementation for x86_64 on is quite straightforward. The code analyses the assembler located after the return address of objc_autoreleaseReturnValue for the presence of a call to objc_retainAutoreleasedReturnValue.

But for ARM:

It looks like the code is identifying the presence of objc_retainAutoreleasedReturnValue not by looking up the presence of a call to that specific function, but by looking instead for a special no-op operation mov r7, r7.

Bill Bumgarner:

ARM’s addressing modes don’t really allow for direct addressing across the full address space. The instructions used to do addressing -- loads, stores, etc… -- don’t support direct access to the full address space as they are limited in bit width.

Greg Parker:

A resolved dyld stub is simple on Intel: it’s just a branch to a branch. On ARM the instruction sequences for the branch to the stub and the branch from the stub can take many different forms depending on how long the branches are. Checking for each combination would be slow.

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