c char translated to i8 signext

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1

I have a c function that takes a char as a parameter. When it is compiled to bytecode, it gets translated to i8 signext. Why is signext getting added to the type? It doesn't get added if the parameter is an int. Is there a way to alter the parameter in the c code so that it simply gets translated to an i8, or is there a way in LLVM to modify the parameter's type?

By the way, I am using LLVM 2.1 in case this has been changed since then.

Regards,
Ryan

2

Ryan:

Not sure if this helps, but...

In the absence of an explicit signed/unsigned qualifier, the ANSI C
standard states that the signedness of char is implementation defined.

In the absence of a prototype, the compiler is obliged to pass that char
as a word, which (if char is signed) requires sign extend.

In the presence of a prototype, the sign extend is probably not strictly
required, but doing it regardless is a partial defense against certain
classes of common programming errors (not sure if this is what is going
on here).

So: if you don't have a prototype visible at the call site, add one and
see what happens.

shap

3

This is an ABI requirement for your target. You can remove it from the llvm ir by removing the parameter attribute from the function and calls to it.

-Chris

4

Thanks for your response. When I attempt to get the parameter attribute lists for the function and its call sites, the list is NULL. Is there someone else I should be looking to get the parameter attributes?

Chris Lattner wrote:

5

It appears that the only thing that has parameter attributes is the function type. However, you can't simply change a function's type without reconstructing the whole function, can you? Also, am I correct that it would not be safe to remove the parameter attribute's from a FunctionType?

Ryan M. Lefever wrote:

6

It appears that the only thing that has parameter attributes is the
function type. However, you can't simply change a function's type
without reconstructing the whole function, can you? Also, am I correct
that it would not be safe to remove the parameter attribute's from a
FunctionType?

Update to llvm 2.2 or mainline, there the attributes are on the function and call for precisely that reason.

-Chris

Ryan M. Lefever wrote:

Thanks for your response. When I attempt to get the parameter attribute
lists for the function and its call sites, the list is NULL. Is there
someone else I should be looking to get the parameter attributes?

Chris Lattner wrote:

I have a c function that takes a char as a parameter. When it is
compiled to bytecode, it gets translated to i8 signext. Why is signext
getting added to the type? It doesn't get added if the parameter is an
int. Is there a way to alter the parameter in the c code so that it
simply gets translated to an i8, or is there a way in LLVM to modify the
parameter's type?

This is an ABI requirement for your target. You can remove it from the
llvm ir by removing the parameter attribute from the function and calls to
it.

-Chris

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_______________________________________________
LLVM Developers mailing list
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http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev

-Chris