Re: A microoptimization of isnegative or greaterthan2millions.

Re: A microoptimization of isnegative or greaterthan2millions.

[J.C. Pizarro]

I'm sorry, i did want to say 2 billions, not 2 millions.

J.C. Pizarro

A microoptimization of isnegative or greaterthan2millions.

[J.C. Pizarro]

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/* Given X an unsigned of 32 bits, and Y a bool. Try to translate optimizing
 *
 * Y = X >  2147483647;   to   Y = ((signed)X) < 0;
 * Y = X >= 2147483648;   to   Y = ((signed)X) < 0;
 *
 * [ Another optimization is to Y = (X >> 31) ]
 *
 * The opposite (ELSE):
 *
 * Y = X <= 2147483647;   to   Y = ((signed)X) >= 0;
 * Y = X <  2147483648;   to   Y = ((signed)X) >= 0;
 *
 * [ Another optimization is to Y = ((~X) >> 31) ]
 *
 * 2147483647=0x7FFFFFFF   2147483648=0x80000000
 *
 * The unsigned comparison is become to signed comparison.
 *
 * by J.C. Pizarro */

#include <stdio.h>

/* isnegative means greaterthan2millions */

int isnegative_1(unsigned int X) {
   int result; // Y is the conditional expression of if-else.
   if (X > 2147483647) result = 1;
   else                result = 0;
   return result;
}

int isnegative_2(unsigned int X) {
   int result; // Y is the conditional expression of if-else.
   if (X >= 2147483648U) result = 1;
   else                  result = 0;
   return result;
}

int isnegative_3(unsigned int X) {
   int result; // Y is the conditional expression of if-else.
   if (X <= 2147483647) result = 0;
   else                 result = 1;
   return result;
}

int isnegative_4(unsigned int X) {
   int result; // Y is the conditional expression of if-else.
   if (X < 2147483648U) result = 0;
   else                 result = 1;
   return result;
}

int isnegative_optimized_1(unsigned int X) {
   int result; // Y is the conditional expression of if-else.
   if (((signed)X) < 0) result = 1;
   else                 result = 0;
   return result;
}

int isnegative_optimized_2(unsigned int X) {
   int result; // Y is the conditional expression of if-else.
   if (((signed)X) >= 0) result = 0;
   else                  result = 1;
   return result;
}

int isnegative_optimized_3(unsigned int X) {
   int result; // Y is the conditional expression of if-else.
   if (X >> 31) result = 1;
   else         result = 0;
   return result;
}

int isnegative_optimized_4(unsigned int X) {
   int result; // Y is the conditional expression of if-else.
   if ((~X) >> 31) result = 0;
   else            result = 1;
   return result;
}

int are_equivalent_isnegative(unsigned int X) {
   int equiv=1,isneg;
   isneg = isnegative_1(X);
   equiv = equiv && (isnegative_2(X) == isneg);
   equiv = equiv && (isnegative_3(X) == isneg);
   equiv = equiv && (isnegative_4(X) == isneg);
   equiv = equiv && (isnegative_optimized_1(X) == isneg);
   equiv = equiv && (isnegative_optimized_2(X) == isneg);
   equiv = equiv && (isnegative_optimized_3(X) == isneg);
   equiv = equiv && (isnegative_optimized_4(X) == isneg);
   return equiv;
}

int main(int argc,char *argv[]) {
   long long X;
   int testOK=1;
   for (X=0LL;(X<=0x0FFFFFFFFLL)&&testOK;X++) {
      testOK = are_equivalent_isnegative((unsigned int)(X&0xFFFFFFFF));
   }
   if (testOK) printf("Full test of isnegative is PASSED.\n");
   else        printf("Full test of isnegative is FAILED.\n");
   return 0;
}

------------------------------------------------------------------------------
# gcc version 4.1.3 20070326 (prerelease)
Full test of isnegative is PASSED.

		     notl    %eax
		     shrl    $31, %eax
		     xorl    $1, %eax

		     IS WORSE THAN

		     shrl    $31, %eax

---------------------

		     xorl    %eax, %eax
		     cmpl    $0, 4(%esp)
		     sets    %al

		     IS WORSE THAN

		     movl    4(%esp), %eax
		     shrl    $31, %eax

------------------------------------------------------------------------------

J.C. Pizarro

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Re: A microoptimization of isnegative or greaterthan2millions.

[Ian Lance Taylor]

"J.C. Pizarro" <jcpiza@gmail.com> writes:

> /* Given X an unsigned of 32 bits, and Y a bool. Try to translate optimizing
>  *
>  * Y = X >  2147483647;   to   Y = ((signed)X) < 0;
>  * Y = X >= 2147483648;   to   Y = ((signed)X) < 0;
>  *
>  * [ Another optimization is to Y = (X >> 31) ]


As far as I can tell, you are recommending that gcc generate a
different code sequence than it currently does.  The most helpful
approach you can use for such a suggestion is to open a bug report
marked as an enhancement.  See http://gcc.gnu.org/bugs.html.

Postings to gcc@gcc.gnu.org are not wrong, but they will almost
certainly get lost.  An entry in the bug database will not get lost.

Thanks.

Ian

Re: A microoptimization of isnegative or greaterthan2millions.

[J.C. Pizarro]

10 Apr 2007 10:53:08 -0700, Ian Lance Taylor <iant@google.com> wrote:
> As far as I can tell, you are recommending that gcc generate a
> different code sequence than it currently does.  The most helpful
> approach you can use for such a suggestion is to open a bug report
> marked as an enhancement.  See http://gcc.gnu.org/bugs.html.
>
> Postings to gcc@gcc.gnu.org are not wrong, but they will almost
> certainly get lost.  An entry in the bug database will not get lost.
>
> Thanks.
>
> Ian
>
Thanks, bug reported as enhancement in
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=31531