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def3ec5689
On win32, NS_InvokeByIndex is implemented with inline assembly. This
inline assembly assumes that it is wrapped by the compiler with the
standard x86 prologue and epilogue:
push ebp
mov ebp, esp
[inline assembly that manipulates the stack pointer]
pop ebp
ret
In particular, the last instruction of the inline assembly is:
mov esp, ebp
which cancels out the effects of the stack manipulation performed by all
the inline assembly that proceeds the instruction.
When compiling with clang-cl, however, the above assumption does not
hold, as clang-cl inserts a more complex prologue and epilogue,
something like:
push ebp
mov ebp, esp
sub esp, frame_size
[save registers into stack frame]
[inline assembly that manipulates the stack pointer]
[restore registers from stack frame]
add esp, frame_size
mov esp, ebp
pop ebp
ret
Combining this more extensive prologue and epilogue with the assumptions
of the inline assembly leads to interesting crashes when
NS_InvokeByIndex is called: the inline assembly effectively deallocates
the stack allocated by the inline assembly *and* the stack frame
allocated by the compiler itself. The compiler-generated code then
attemptes to deallocate the stack frame, leading to the crash, as the
code now returns to an unspecified address.
To avoid these sorts of problems in clang-cl and make the code more
robust generally, let's move the NS_InvokeByIndex implementation to a
separate assembly file. We can then write exactly what we need to have
happen, safe from any manipulations of the compiler.
Since we don't compile much (any?) code in Gecko with MASM, we need to
add the /SAFESEH flag to the assembler invocation so that the object
file with be appropriately marked as not containing exception handlers;
the linker (which is invoked with the /SAFESEH flag itself) will then
consent to link it into libxul.
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