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.
