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[safestack] Fast access to the unsafe stack pointer on AArch64/Android.

Description

[safestack] Fast access to the unsafe stack pointer on AArch64/Android.

Android libc provides a fixed TLS slot for the unsafe stack pointer,
and this change implements direct access to that slot on AArch64 via
__builtin_thread_pointer() + offset.

This change also moves more code into TargetLowering and its
target-specific subclasses to get rid of target-specific codegen
in SafeStackPass.

This change does not touch the ARM backend because ARM lowers
builting_thread_pointer as aeabi_read_tp, which is not available
on Android.

The previous iteration of this change was reverted in r250461. This
version leaves the generic, compiler-rt based implementation in
SafeStack.cpp instead of moving it to TargetLoweringBase in order to
allow testing without a TargetMachine.

Details

Committed
eugenisOct 26 2015, 11:28 AM
Parents
rL251323: BitstreamWriter: Fix integer overflow.
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