[Bug optimization/13958] New: Conversion from unsigned to double is painfully slow on P4

[reichelt at gcc dot gnu dot org]

Conversion from unsigned to double is painfully slow on P4.
(I don't know whether other x86 architectures are affected as well.)
Just consider the following program:

==================================================
unsigned a[2]={1,2};

inline unsigned foo1(int i) { return a[i]; }
inline int      foo2(int i) { return a[i]; }

int main()
{
    double x=0;
    int    i;

    for ( i=0; i<100000000; ++i )
        x+=foo1(i%2);

    return (int)x;
}
==================================================

Compiled with "-O2" the program executes in about 2s on a 3 GHz P4.
If I use "foo2" instead of "foo1" in the argument of "x+=",
the execution time drops by a factor of 10 to 0.2s!

This happens with all versions since gcc 2.95.x.

Looking at the assembler it seems that we convert the unsigned to a
long long variable which is then converted to a double:

	xorl	%edx, %edx
	pushl	%edx
	pushl	%eax
	fildll	(%esp)

Maybe even using only "fildl" and adding the double constant 2^32 later
to the result if the highest bit of the operand was set might be faster.


Btw, such conversions are quite common in numerical codes that deal
with uniform grids: the array index can be used as a coordinate (usually
after some trivial scaling). Given that the indices used in libstdc++
are usually of the type size_t the slow conversion can have quite a
negative performance impact.

-- 
           Summary: Conversion from unsigned to double is painfully slow on
                    P4
           Product: gcc
           Version: 3.4.0
            Status: UNCONFIRMED
          Keywords: pessimizes-code
          Severity: normal
          Priority: P2
         Component: optimization
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: reichelt at gcc dot gnu dot org
                CC: gcc-bugs at gcc dot gnu dot org
  GCC host triplet: i686-pc-linux-gnu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=13958