[LLVMdev] llvm_fcmp_ord and llvm_fcmp_uno and assembly code generation

[Evan Cheng]

Hi,

Can you file a bugzilla on this? Thanks!

Evan

On Oct 19, 2007, at 3:50 AM, Török Edvin wrote:

> 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
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