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- [/
- Copyright Oliver Kowalke 2014.
- Distributed under the Boost Software License, Version 1.0.
- (See accompanying file LICENSE_1_0.txt or copy at
- http://www.boost.org/LICENSE_1_0.txt
- ]
- [section:rationale Rationale]
- [heading No inline-assembler]
- Some newer compiler (for instance MSVC 10 for x86_64 and itanium) do not
- support inline assembler.
- [footnote [@http://msdn.microsoft.com/en-us/library/4ks26t93.aspx MSDN article
- 'Inline Assembler']].
- Inlined assembler generates code bloating which is not welcome on embedded
- systems.
- [heading fcontext_t]
- __boost_context__ provides the low level API fcontext_t which is
- implemented in assembler to provide context swapping operations.
- fcontext_t is the part to port to new platforms.
- [note Context switches do not preserve the signal mask on UNIX systems.]
- __fcontext__ is an opaque pointer.
- [section Other APIs ]
- [heading setjmp()/longjmp()]
- C99 defines `setjmp()`/`longjmp()` to provide non-local jumps but it does not
- require that ['longjmp()] preserves the current stack frame. Therefore, jumping
- into a function which was exited via a call to ['longjmp()] is undefined
- [footnote ISO/IEC 9899:1999, 2005, 7.13.2.1:2].
- [#ucontext]
- [heading ucontext_t]
- Since POSIX.1-2004 `ucontext_t` is deprecated and was removed in POSIX.1-2008!
- The function signature of `makecontext()` is:
- void makecontext(ucontext_t *ucp, void (*func)(), int argc, ...);
- The third argument of `makecontext()` specifies the number of integer arguments
- that follow which will require function pointer cast if `func` will accept those
- arguments which is undefined in C99
- [footnote ISO/IEC 9899:1999, 2005, J.2].
- The arguments in the var-arg list are required to be integers, passing pointers
- in var-arg list is not guaranteed to work, especially it will fail for
- architectures where pointers are larger than integers.
- `ucontext_t` preserves signal mask between context switches which involves system
- calls consuming a lot of CPU cycles (ucontext_t is slower; a context switch
- takes [link performance ['two magnitutes of order more CPU cycles]] more than
- __fcontext__).
- [heading Windows fibers]
- A drawback of Windows Fiber API is that `CreateFiber()` does not accept a
- pointer to user allocated stack space preventing the reuse of stacks for other
- context instances. Because the Windows Fiber API requires to call
- `ConvertThreadToFiber()` if `SwitchFiber()` is called for a thread which has not
- been converted to a fiber. For the same reason `ConvertFiberToThread()`
- must be called after return from `SwitchFiber()` if the thread was forced to be
- converted to a fiber before (which is inefficient).
- if ( ! is_a_fiber() )
- {
- ConvertThreadToFiber( 0);
- SwitchToFiber( ctx);
- ConvertFiberToThread();
- }
- If the condition `_WIN32_WINNT >= _WIN32_WINNT_VISTA` is met function
- `IsThreadAFiber()` is provided in order to detect if the current thread was
- already converted. Unfortunately Windows XP + SP 2/3 defines
- `_WIN32_WINNT >= _WIN32_WINNT_VISTA` without providing `IsThreadAFiber()`.
- [endsect]
- [section x86 and floating-point env]
- [heading i386]
- "The FpCsr and the MxCsr register must be saved and restored before any call or return
- by any procedure that needs to modify them ..."
- [footnote 'Calling Conventions', Agner Fog].
- [heading x86_64]
- [heading Windows]
- MxCsr - "A callee that modifies any of the non-volatile fields within MxCsr must restore
- them before returning to its caller. Furthermore, a caller that has modified any
- of these fields must restore them to their standard values before invoking a callee ..."
- [footnote [@http://http://msdn.microsoft.com/en-us/library/yxty7t75.aspx MSDN article
- 'MxCsr']].
- FpCsr - "A callee that modifies any of the fields within FpCsr must restore them before
- returning to its caller. Furthermore, a caller that has modified any of these
- fields must restore them to their standard values before invoking a callee ..."
- [footnote [@http://http://msdn.microsoft.com/en-us/library/ms235300.aspx MSDN article
- 'FpCsr']].
- "The MMX and floating-point stack registers (MM0-MM7/ST0-ST7) are preserved across
- context switches. There is no explicit calling convention for these registers."
- [footnote [@http://msdn.microsoft.com/en-us/library/a32tsf7t%28VS.80%29.aspx MSDN article
- 'Legacy Floating-Point Support']].
- "The 64-bit Microsoft compiler does not use ST(0)-ST(7)/MM0-MM7".
- [footnote 'Calling Conventions', Agner Fog].
- "XMM6-XMM15 must be preserved"
- [footnote [@http://msdn.microsoft.com/en-us/library/9z1stfyw%28v=vs.100%29.aspx MSDN
- article 'Register Usage']]
- [heading SysV]
- "The control bits of the MxCsr register are callee-saved (preserved across calls),
- while the status bits are caller-saved (not preserved). The x87 status word register is
- caller-saved, whereas the x87 control word (FpCsr) is callee-saved."
- [footnote SysV ABI AMD64 Architecture Processor Supplement Draft Version 0.99.4, 3.2.1].
- [endsect]
- [endsect]
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