[cfe-dev] __builtin_constant_p and floating-point constants

[Jonathan Sauer]

Hello,

please consider the following (not very useful) program:

inline int foo(float a, int b)
{
	if (__builtin_constant_p(a))
		return int(a) * b;
	else
		return a * b;
}


int bar(int b)
{
	return foo(23.0f, b);
}


int main(int, char**)
{
}


If <a> is a constant when <foo> is called, only its integer part should be multiplied with <b>, resulting
in an integer multiplication. If <a> is not constant, <b> should be converted to a float, multiplied with
<a> and truncated back to an <int> instead.

Apple's GCC 4.2.1 handles this as expected ("gcc RandomTest.cpp -S -O3 -o -", small cleanups of the output):

	pushq	%rbp
	movq	%rsp, %rbp
	imull	$23, %edi, %eax
	leave
	ret


Clang r132676 on the other hand ("clang RandomTest.cpp -S -O3 -o -", again slightly cleaned output):

	pushq	%rbp
	movq	%rsp, %rbp
	cvtsi2ss	%edi, %xmm0
	mulss	LCPI0_0(%rip), %xmm0
	cvttss2si	%xmm0, %eax
	popq	%rbp
	ret


It seems that clang does not evaluate __builtin_constant_p in the same way as GCC does. Is this a bug
or intentional? (personally, I would think the latter, as there does not seem to exist a reason why
__builtin_constant_p should not work with floating-point constants)


With many thanks in advance,
Jonathan

P.S:: Apple's LLVM-GCC 4.2 produces the same result as clang.