[Buildroot] Xtensa toolchain issue ?
thomas.petazzoni at bootlin.com
Thu Jan 10 14:56:49 UTC 2019
On Fri, 4 Jan 2019 15:44:36 -0800, Max Filippov wrote:
> A thunk function is used to implement C++ virtual function calls with
> multiple inheritance. Probably the best explanation is a quote from
> the gccint:
> These functions represent stub code that adjusts the 'this' pointer
> and then jumps to another function. When the jumped-to function
> returns, control is transferred directly to the caller, without
> returning to the thunk. The first parameter to the thunk is always
> the 'this' pointer; the thunk should add 'THUNK_DELTA' to this
> value. (The 'THUNK_DELTA' is an 'int', not an 'INTEGER_CST'.)
> Then, if 'THUNK_VCALL_OFFSET' (an 'INTEGER_CST') is nonzero the
> adjusted 'this' pointer must be adjusted again. The complete
> calculation is given by the following pseudo-code:
> this += THUNK_DELTA
> if (THUNK_VCALL_OFFSET)
> this += (*((ptrdiff_t **) this))[THUNK_VCALL_OFFSET]
> Finally, the thunk should jump to the location given by
> 'DECL_INITIAL'; this will always be an expression for the address
> of a function.
> and then later, in the description of macro
> If you do not define this macro, the target-independent code in the
> C++ front end will generate a less efficient heavyweight thunk that
> calls FUNCTION instead of jumping to it. The generic approach does
> not support varargs.
Thanks for the additional explanation, very useful!
> > Since the issue also happens on OpenRISC, perhaps we need to have some
> > BR2_TOOLCHAIN_SUPPORTS_XYZ boolean that says whether the gcc version
> > supports this specific stuff. However, since I don't clearly understand
> > what the issue is, I wouldn't be able to find a good name for that
> > BR2_TOOLCHAIN_SUPPORTS_XYZ option.
> > BR2_TOOLCHAIN_SUPPORTS_TAIL_CALL_VARIADIC_FUNC_FROM_THUNK ? :-)
> Maybe BR2_TOOLCHAIN_SUPPORTS_VARIADIC_MI_THUNK?
Sounds like a good name. Do you want to cook a patch for this, or
should I do so ?
Thomas Petazzoni, CTO, Bootlin
Embedded Linux and Kernel engineering
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