Problem with egcs and denormalized constants?
Randy Gobbel
gobbel at andrew.cmu.edu
Fri Mar 5 09:27:09 EST 1999
"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 at 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 at 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?
It's pretty easy to see what's going on if you look at it with gdb.
(gdb) x/10 0x1802078
0x1802078 <.LC0>: 1.40239843e-37 1.40239834e-38 1.40239848e-39
1.40239148e-40
0x1802088 <.LC4>: 1.4024195e-41 1.40269976e-42 1.40129846e-43
1.40129846e-44
0x1802098 <.LC8>: 0 2.79124188e+23
(gdb) x/10x 0x1802078
0x1802078 <.LC0>: 0x023ee274 0x0098b529 0x000f4551
0x000186ee
0x1802088 <.LC4>: 0x00002718 0x000003e9 0x00000064
0x0000000a
0x1802098 <.LC8>: 0x00000000 0x666c6d69
Sure looks like an egcs bug to me. The constant is being generated
incorrectly--it should show up as a 1 in hex. If I manually stuff a 1
into .LC8, the program prints out the correct number:
(gdb) x/10x 0x1802078
0x1802078 <.LC0>: 0x023ee274 0x0098b529 0x000f4551
0x000186ee
0x1802088 <.LC4>: 0x00002718 0x000003e9 0x00000064
0x0000000a
0x1802098 <.LC8>: 0x00000001 0x666c6d69
(gdb) c
Continuing.
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
-Randy
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