[LLVMdev] llvm_fcmp_ord and llvm_fcmp_uno and assembly code generation

[Török Edvin]

Hi,

The C backend in llc generates code like:
static inline int llvm_fcmp_ord(double X, double Y) { return X == X && Y == Y; }
static inline int llvm_fcmp_uno(double X, double Y) { return X != X || Y != Y; }

First of all it generates a warning by clang and gcc (with certain flags):
x.cbe.c:130: warning: comparing floating point with == or != is unsafe

Now, C99 provides a macro for this kind of stuff, but unfortunately
ANSI C doesn't have something like this (for unordered testing) AFAIK.

*If* we would be using C99 the code could look like:
return isunordered(X, Y);
return !isunordered(X, Y);

However the assembly code generated is much shorter if I am using the
C99 macros, both on gcc and llvm-gcc.

This raises 2 issues:
* can llvm_fcmp_ord/uno be implemented in ANSI/ISO C differently,
which doesn't generate a warning, *and* generates optimal code
* can llvm-gcc be improved to recognize functions like
llvm_fcmp_ord/uno, and generate the optimal code (one ucomisd, rather
than two).

Not that llvm_fcmp_ord/uno would be on a critical path in a program,
but any optimization
is good, and worth mentioning IMHO ;)

Look:
#include <math.h>
static inline int llvm_fcmp_ord(double X, double Y) { return X == X && Y == Y; }
static inline int llvm_fcmp_uno(double X, double Y) { return X != X || Y != Y; }
int x(double X, double Y)
{
	return llvm_fcmp_uno(X,Y);
}

int xx(double X, double Y)
{
	return isunordered(X, Y);
}

$  gcc -std=c99 -O3 -S x.c -o x.gcc.s
$  llvm-gcc -std=c99 -O3 -S x.c -o x.llvm.s

x.gcc.s:
x:
.LFB7:
	movl	$1, %eax
	ucomisd	%xmm0, %xmm0
	jne	.L5
	jp	.L5
	xorl	%eax, %eax
	ucomisd	%xmm1, %xmm1
	setp	%al
.L5:
	rep ; ret
.LFE7:
	.size	x, .-x
	.p2align 4,,15
.globl xx
	.type	xx, @function
xx:
.LFB8:
	xorl	%eax, %eax
	ucomisd	%xmm1, %xmm0
	setp	%al
	ret

x.llvm.s:
x:
	pxor	%xmm2, %xmm2
	ucomisd	%xmm2, %xmm0
	setp	%al
	ucomisd	%xmm2, %xmm1
	setp	%cl
	orb	%al, %cl
	movzbl	%cl, %eax
	ret
	.size	x, .-x


	.align	16
	.globl	xx
	.type	xx, at function
xx:
	ucomisd	%xmm1, %xmm0
	setp	%al
	movzbl	%al, %eax
	ret
	.size	xx, .-xx

Best regards,
Edwin