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x86inc: Avoid using eax/rax for storing the stack pointer 

When allocating stack space with an alignment requirement that is larger
than the current stack alignment we need to store a copy of the original
stack pointer in order to be able to restore it later.

If we chose to use another register for this purpose we should not pick
eax/rax since it can be overwritten as a return value.
This commit is contained in:
Henrik Gramner 2016-12-20 21:33:12 +01:00
parent 0e8b68a2c4
commit cd09e3b349
1 changed files with 7 additions and 0 deletions
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7
libavutil/x86/x86inc.asm
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@ -385,7 +385,14 @@ DECLARE_REG_TMP_SIZE 0,1,2,3,4,5,6,7,8,9,10,11,12,13,14
%ifnum %1
%if %1 != 0 && required_stack_alignment > STACK_ALIGNMENT
%if %1 > 0
; Reserve an additional register for storing the original stack pointer, but avoid using
; eax/rax for this purpose since it can potentially get overwritten as a return value.
%assign regs_used (regs_used + 1)
%if ARCH_X86_64 && regs_used == 7
%assign regs_used 8
%elif ARCH_X86_64 == 0 && regs_used == 1
%assign regs_used 2
%endif
%endif
%if ARCH_X86_64 && regs_used < 5 + UNIX64 * 3
; Ensure that we don't clobber any registers containing arguments. For UNIX64 we also preserve r6 (rax)