gmane.comp.parsers.sparse

David Given | 6 Sep 23:11

Odd behaviour with OP_SCAST

test-unssa.c compiles this:

extern void func(int i, ...);
void foo(int i) { func(0, (float) i, (double) i, i); }

...into this:

	fpcast.64   %r3 <- (32) %arg1
	scast.64    %r5 <- (32) %arg1
	call        func, $0, %r3, %r5, %arg1
	ret

That's with the default settings. Unfortunately, with Clue,
sizeof(double) == sizeof(int), so that second instruction comes out as:

	scast.32    %r5 <- (32) %arg1

This then causes the simplification code in simplify_cast() to discard it:

	if (size == orig_size) {
		int op = (orig_type->ctype.modifiers & MOD_SIGNED) ? OP_SCAST : OP_CAST;
		if (insn->opcode == op)
			goto simplify;
	}

The end result is that my call statement turns into:

	call        func, $0, %r3, %arg1, %arg1

...which is wrong.

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headers

Chris Li | 8 Sep 23:02

Re: Odd behaviour with OP_SCAST

On Sat, Sep 6, 2008 at 2:14 PM, David Given <dg <at> cowlark.com> wrote:
> The end result is that my call statement turns into:
>
>        call        func, $0, %r3, %arg1, %arg1
>
> ...which is wrong.
>
> I assume that Clue's odd configuration is violating some assumption
> somewhere, but I'm not well-enough versed with the sparse internals to
> know where. It does seem odd to me that it's generating an OP_SCAST to
> convert the int to a double, rather than an OP_FPCAST like in the float.

Sparse assume that float and double have different size than int.
On the linearized instruction level, it does not have the full type
information any more. All it got is the size of the type. So it is
tricky to distinguish int vs float.

It generate cast to double because your third argument is a
double type.

Because the kernel does not have any floating pointer
code. Sparse did not take floating point very seriously.

The change require the instruction to have more type
information than size. I am tempting to just put a full
ctype pointer there.

Chris
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headers

David Given | 10 Sep 22:48

Re: Odd behaviour with OP_SCAST

Chris Li wrote:
[...]
> Sparse assume that float and double have different size than int.
> On the linearized instruction level, it does not have the full type
> information any more. All it got is the size of the type. So it is
> tricky to distinguish int vs float.

Here's another one; and this is with a stock unmodified sparse, too, so
I know it's not my fault.

extern double pow(double x, double y);
double d(double p)
{ return pow(2, p); }

-->

	call.64     %r2 <- pow, $2, %arg1
	ret.64      %r2

(Again with test-unssa, which seems to be convenient for displaying
intermediate code.)

[...]
> The change require the instruction to have more type
> information than size. I am tempting to just put a full
> ctype pointer there.

In fact, that's what I've got in the private patch I'm using for Clue,
although I'm only using it for determining the types of pseudos; I just
hacked alloc_typed_instruction() (which knows the type) to add it. No

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