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John Miles
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 Posted: Tue Jul 29, 2003 6:09 pm Post subject: Compiler's use of function parameters as temporaries |
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Hi --
This is a bit of an implementation-specific problem, but I'd like to
post it here to see if there's a general answer within the auspices of
the language.
I'm developing a high(er)-level scripting language designed explicitly
to complement C++. One of the language's features is the ability to
pass "out" parameters to an MSVC __cdecl function by reference. After
the C++ function returns, but before popping the parameters from the
stack, the scripting-language VM copies any out-parameters back from the
C++ stack frame to the variables they originally came from. In this way
any modifications made by the C++ function to its parameters are
propagated back to the scripting language.
As you might guess, this approach has problems when compiling with
optimization turned on. In theory the C++ compiler may decide that any
write operations to parameters which are never subsequently read can be
omitted. In practice, I haven't seen that happening, but what I *have*
seen is that the compiler likes to reuse function parameters when it
needs to allocate a temporary. Hilarity ensues when the VM retrieves an
unrelated temporary from the stack instead of the parameter it expects.
Is there a general, implementation-independent way to tell the compiler
(a) not to reuse a function parameter as a temporary; and (b) not to
perform dead-code elimination on writes to function parameters?
I experimentally tried applying "volatile" to a parameter ("volatile
Mat4x4 *SRC"), and while MSVC accepted the code, SRC still got reused as
a temporary within the body of the function.
Similarly, passing the parameters by reference rather than as pointers
("Mat4X4 &SRC") doesn't make any difference.
Thanks in advance for any suggestions.
-- jm
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http://www.qsl.net/ke5fx
Note: My E-mail address has been altered to avoid spam
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tom_usenet
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| Posted: Wed Jul 30, 2003 1:33 pm Post subject: Re: Compiler's use of function parameters as temporaries |
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On Tue, 29 Jul 2003 11:09:18 -0700, John Miles
<jmiles (AT) pop (DOT) removethistomailme.net> wrote:
| Quote: | Hi --
This is a bit of an implementation-specific problem, but I'd like to
post it here to see if there's a general answer within the auspices of
the language.
I'm developing a high(er)-level scripting language designed explicitly
to complement C++. One of the language's features is the ability to
pass "out" parameters to an MSVC __cdecl function by reference.
|
How are the parameters in the C++ function declared? If parameters are
passed by value, then the compiler is highly likely to modify them
arbitrarily inside the function. In addition, the function might even
operate on a copy of the parameter, so that changes aren't accessible
in the calling stack frame.
After
| Quote: | the C++ function returns, but before popping the parameters from the
stack, the scripting-language VM copies any out-parameters back from the
C++ stack frame to the variables they originally came from. In this way
any modifications made by the C++ function to its parameters are
propagated back to the scripting language.
As you might guess, this approach has problems when compiling with
optimization turned on. In theory the C++ compiler may decide that any
write operations to parameters which are never subsequently read can be
omitted. In practice, I haven't seen that happening, but what I *have*
seen is that the compiler likes to reuse function parameters when it
needs to allocate a temporary. Hilarity ensues when the VM retrieves an
unrelated temporary from the stack instead of the parameter it expects.
Is there a general, implementation-independent way to tell the compiler
(a) not to reuse a function parameter as a temporary; and (b) not to
perform dead-code elimination on writes to function parameters?
|
If the parameter is passed by value there is no reason for the
compiler to leave the variable in the state in which it was passed,
and I doubt there is a way to make it.
| Quote: |
I experimentally tried applying "volatile" to a parameter ("volatile
Mat4x4 *SRC"), and while MSVC accepted the code, SRC still got reused as
a temporary within the body of the function.
Similarly, passing the parameters by reference rather than as pointers
("Mat4X4 &SRC") doesn't make any difference.
|
It sounds like you are doing the wrong thing. You need to have the C++
function take pointers, not values. References might work, bearing in
mind that references are probably implemented as pointers.
Tom
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