Bug#381710: gcc-4.1: wrong code generation for perl on hppa/mips.

[Kurt Roeckx <kurt@roeckx.be>]

Package: gcc-4.1
Version: 4.1.1-9

I've been looking at the perl testsuite failure on hppa.  See
http://bugs.debian.org/374396

This code:
	while (cdouble < 0.0)
		cdouble += adouble;

Generated by gcc-4.1 with -O2 and -fdelayed-branch gives:
        fadd,dbl %fr13,%fr22,%fr13
 .L1447:
        fcmp,dbl,!< %fr13,%fr0
        ftest
        b .L1447
        fadd,dbl %fr13,%fr22,%fr13
        fsub,dbl %fr13,%fr22,%fr13

With -O2 and -fno-delayed-branch:
.L1239:
        fadd,dbl %fr13,%fr22,%fr13
        fcmp,dbl,!< %fr13,%fr0
        ftest
        b,n .L1239

As you can see, in case of the delayed branches it always
executes an fadd at the start and fsub at the end, which it
doesn't do without the delayed branches.

This is causing unwanted rounding problems, since the mantisa
doesn't have enough bits to keep the the required information.
I think atleast in this case, it's not a good idea to do this
optimization with floating point numbers.

The same code on gcc-4.0 with -fdelayed-branch seems to generate
this code:
.L661:
        fadd,dbl %fr12,%fr22,%fr12
        fcmp,dbl,!< %fr12,%fr0
        ftest
        b .L661
        ldo -256(%r30),%r20

With -fno-delayed-branch:
.L643:
        fadd,dbl %fr12,%fr22,%fr12
        fcmp,dbl,!< %fr12,%fr0
        ftest
        b,n .L643

So gcc-4.0 looks good.

gcc-snapshot 20060721-1 gives with -fdelayed-branch:
        fadd,dbl %fr12,%fr22,%fr12
.L1449:
        fcmp,dbl,!< %fr12,%fr0
        ftest
        b .L1449
        fadd,dbl %fr12,%fr22,%fr12
        fsub,dbl %fr12,%fr22,%fr12

So that has the same problem.


For those not familiar with hppa assembler, a branch normally
executes the instruction following it too, before branching.
The ",n" in "b,n" will prevent the next instruction from being
executed, so has the same effect as following it with a nop
instruction.

The following code has the same effect:
#include <stdio.h>
double cdouble = -1;

int     main()
{
        double adouble;

        adouble = 9007199254740992.0; /* 2^53 */
        while (cdouble < 0.0)
                cdouble += adouble;
        printf("%lf\n", cdouble);
        return 0;
}

With delayed branches it prints:
9007199254740992.000000
without:
9007199254740991.000000


Kurt