Problem with egcs and denormalized constants?

[Gary Thomas]

This turns out to be a bug in the GAS IEEE FP code.  It could potentially
happen on any platform, but it is seldom seen from C code.  This is because
most of the GCC/EGCS back-ends generate raw hex values for the FP constants.
The PowerPC back-end uses the ".float" directive instead.

Anyway, I have a patch for GAS which fixes it.  I'll be making a new set of
binutils RPMS (rev 19b) which should be available later today at:
  ftp://ftp.linuxppc.org/linuxppc/users/gdt/redhat/RPMS/ppc/

On 04-Mar-99 Kevin B. Hendricks wrote:
> Hi,
> 
> The LinuxPPC port of JDK 1.2 can't pass the Java Compatibility Kit runtime-vm
> tests because of some sort of error which is related to having very small
> de-nromalized float constants.
> 
> I don't know whether this is an egcs problem (it happens with both egcs 1.1.1
> and egcs 1.0.2), glibc problem (tested with the very latest glibc 1.99 rpm from
> Gary) but it is an error.  
> 
> Will someone please compile and try the follwoing very simple test program and
> help me understand what is happening here.  Is this an egcs problem?
> 
> [root@kbhend fltbug]# cat t3.c
>#include <unistd.h>
>#include <stdlib.h>
>#include <string.h>
>#include <stdio.h>
> 
> 
> float flMin (int i)
>  {
>    float fl;
>    if (i == 0) fl = 1.4023984e-37F;
>    if (i == 1) fl = 1.4023984e-38F;
>    if (i == 2) fl = 1.4023984e-39F;
>    if (i == 3) fl = 1.4023984e-40F;
>    if (i == 4) fl = 1.4023984e-41F;
>    if (i == 5) fl = 1.4023984e-42F;
>    if (i == 6) fl = 1.4023984e-43F;
>    if (i == 7) fl = 1.4023984e-44F;
>    if (i == 8) fl = 1.4023984e-45F;
>    return fl;
>  }
> 
> 
> int main(int argc, char** argv)
> {
>   int i;
>   float f;
> 
>   for (i=0;i<9;i++) {
>      f = flMin(i);
>      fprintf(stdout,"flmin(%1d) is %20.13e\n",i,f);
>   }
>   fprintf(stdout,"But flmin(7)/10.0F is %20.13e\n",(flMin(7)/10.0F));
> }
> 
> 
> Here is the output from my LinuxPPC box:
> 
> [root@kbhend fltbug]# ./t3
> flmin(0) is  1.4023984275674e-37
> flmin(1) is  1.4023983434895e-38
> flmin(2) is  1.4023984836193e-39
> flmin(3) is  1.4023914771270e-40
> flmin(4) is  1.4024195030963e-41
> flmin(5) is  1.4026997627891e-42
> flmin(6) is  1.4012984643248e-43
> flmin(7) is  1.4012984643248e-44
> flmin(8) is  0.0000000000000e+00
> But flmin(7)/10.0F is  1.4012984643248e-45            
> 
> 
> Here is the correct output from my AIX box using an old version of gcc:
> 
> kbhend$ gcc -ot3 -O0 t3.c
> kbhend$ ./t3
> flmin(0) is  1.4023984275674e-37
> flmin(1) is  1.4023983434895e-38
> flmin(2) is  1.4023984836193e-39
> flmin(3) is  1.4023914771270e-40
> flmin(4) is  1.4024195030963e-41
> flmin(5) is  1.4026997627891e-42
> flmin(6) is  1.4012984643248e-43
> flmin(7) is  1.4012984643248e-44
> flmin(8) is  1.4012984643248e-45
> But flmin(7)/10.0F is  1.4012984643248e-45
> 
> 
> Notice the difference in the value of flmin(8) when loaded from a constant.
> 
> I looked at the assmebler and the constant for that value is correctly
> identified and present.
> 
> What do you think?
> 
> Kevin

------------------------------------------------------------------------
Gary Thomas                              |
email: gdt@linuxppc.org                  | "Fine wine is a necessity of
   ... opinions expressed here are mine  |        life for me"
       and no one else would claim them! |
                                         |      Thomas Jefferson
------------------------------------------------------------------------