[MIPS] unoptimized __atomic_compare_exchange hangs on Octeon

[llvmbot]

Bugzilla Link	32020
Resolution	FIXED
Resolved on	Oct 12, 2018 04:05
Version	4.0
OS	Linux
Reporter	LLVM Bugzilla Contributor
CC	@orgads,@atanasyan

Extended Description

Hi,

Testcase:

int main(void)
{
volatile int obj = 1;
int x = obj;

__atomic_compare_exchange_n(&obj, &x, 0, 1, __ATOMIC_RELAXED, __ATOMIC_RELAXED);

}

Compiling with:
clang-4.0 -O0 test.c -otes

Running "test" hangs on Cavium Octeon machines. The bug does not occur if -O1 is used.

The generated assembly contains this loop:

.LBB0_1:
ld $1, 16($fp)
ll $2, 0($1)
lw $3, 32($fp)
sw $2, 12($fp)
bne $2, $3, .LBB0_3
nop
lw $1, 28($fp)
ld $2, 16($fp)
sc $1, 0($2)
beqz $1, .LBB0_1
nop

However the Octeon processor manual states that any store will cause a subsequent sc instruction to fail and therefore this above loop will never complete on Octeon.

Thanks,
James

[llvmbot]

Looking into this.

[llvmbot]

This is a occurrence of PR/25526 but for MIPS. I'm currently trialling a fix.

[llvmbot]

Should be resolved by r296134.

[llvmbot]

Thanks! I've tested the fix and it works now.

Is it possible for this to be fixed in either 4.0 or 4.0.1 as well?

[llvmbot]

I've requested it be merged today for 4.0. If not, I'll try and get it in for the next point release of 4.0.

[llvmbot]

The fix posted has problems, reopening this while I work up another solution.

[llvmbot]

The fix posted has problems, reopening this while I work up another solution.

[llvmbot]

Has there been any progress on this bug?

[llvmbot]

I have a fix, but I want to perform more testing before recommitting it. My last attempt fudged handed allocated register, resulting in segfaults everywhere.

[atanasyan]

I have a fix, but I want to perform more testing before recommitting it. My
last attempt fudged handed allocated register, resulting in segfaults
everywhere.

SimonD, do you have the fix (maybe incomplete) mentioned in your comment? Could you please share it?

[llvmbot]

I had a look at this bug and it was resolved in r336328. I recommend closing it.

[atanasyan]

Fixed at https://reviews.llvm.org/rL336328.

[llvmbot]

mentioned in issue llvm/llvm-bugzilla-archive#32102