[Bug libgomp/81778] libgomp.c/for-5.c failure on nvptx -- illegal memory access

["vries at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81778

--- Comment #3 from Tom de Vries <vries at gcc dot gnu.org> ---
The core loop looks like this:
...
 $L4:
  add.u32 %r31,%r42,-1;
  cvt.u64.u32 %r63,%r31;
  shl.b64 %r64,%r63,2;
  add.u64 %r65,%r75,%r64;
  ld.u32 %r71,[%r65];
  add.u32 %r70,%r71,-4;
  st.u32 [%r65],%r70;
  add.u32 %r42,%r42,-32;
  setp.lt.u32 %r72,%r30,%r42;
  @ %r72 bra $L4;
...

If we comment out the branch, the testcase passes.

I've investigated the ranges of the registers to understand why the branch is
taken. [ By storing the reg into the array, and printing it out afterwards. 
That meant disabling the jump, otherwise I don't get to the print. ]:
...
r42: [0,63]  -> [-31,32]
r30: [0-31]  ->  0
     [32-63] -> 32
...

So for index 0, we have:
...
setp.lt.u32 %r72,0,-31;
...

This evaluates to true, because (u32)-31 is bigger than 0.

Changing the compare from unsigned to signed makes the test pass.