[LLVMbugs] [Bug 1738] New: llvm_fcmp_ord and llvm_fcmp_uno and assembly code generation

[bugzilla-daemon at cs.uiuc.edu]

http://llvm.org/bugs/show_bug.cgi?id=1738

           Summary: llvm_fcmp_ord and llvm_fcmp_uno and assembly code
                    generation
           Product: tools
           Version: trunk
          Platform: PC
        OS/Version: Linux
            Status: NEW
          Keywords: code-quality
          Severity: minor
          Priority: P2
         Component: llc
        AssignedTo: unassignedbugs at nondot.org
        ReportedBy: edwintorok at gmail.com
                CC: llvmbugs at cs.uiuc.edu


[previosuly posted on ML]:

Missed optimization in llc, and llvm-gcc.

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


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