[BugFix] [Kernel] Fix GPU SEGV occurring in int8 kernels

[rasmith]

I was running the following model: Phi-3-medium-128k-instruct-quantized.w8a8 and getting GPU SEGV. I found that there was some integer overflow causing a SEGV. An example of how this could happen is:

int a = 131072;
int b = 17920;
std::cout << "a * b = " << a * b << "\n"; // prints -1946157056
std::cout << "((size_t)a) * b = " << ((size_t)a) * b << "\n"; // prints 2348810240
std::cout << " (size_t)(ab) = " << (size_t)(ab) << "\n"; // prints 18446744071763394560

So basically, integer overflow was ending up creating an index that was far too large, e.g. the third cout call. So, if "a" is hidden_size in one of the kernel calls, and "b" is token_idx, the problem can arise.

In the commit, adjusting int8_quant_kernels.cu by making the iterator be of type size_t and changing hidden_size to size_t fixes the issue, ensuring no integer overflow occurs.

FIX #xxxx (link existing issues this PR will resolve)

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[rasmith]

/ready

[mgoin]

cc @tlrmchlsmth @ProExpertProg

[tlrmchlsmth]

Nice catch, I see the issue.

Could you try doing something like this?

  out += static_cast<int64_t>(token_idx) * hidden_size;
  input += static_cast<int64_t>(token_idx) * hidden_size;

  for (int_t i = tid; i < hidden_size; i += blockDim.x) {
    out[i] = float_to_int8_rn(static_cast<float>(input[i]) / scale);
  }

This way, the index for our innermost loop can remain in a 32-bit register. Otherwise, we may see performance implications due to the loop counter taking two registers

[ProExpertProg]

Thanks for submitting this fix! Could you make all indexing variables size_t while we're at it?

[ProExpertProg]

Could you change these to size_t as well? For consistency.

[ProExpertProg]

Same here

[ProExpertProg]

Oops, Tyler's comments just loaded for me - his suggestion is better.

[tlrmchlsmth]

I think hidden_size should still be an int, otherwise e.g. i may be up-converted for comparison

[tlrmchlsmth]

Does it make sense to add a unit test that would have exposed this issue?

[rasmith]

Does it make sense to add a unit test that would have exposed this issue?

@tlrmchlsmth I could do something like this to test_int8_quant.py:

+
+# The last two values in HIDDEN_SIZES and NUM_TOKENS will cause a SEGV on the
+# GPU if integer overflow occurs.
 HIDDEN_SIZES = [16, 67, 768, 2048, 5120, 5137, 8192,
-                8193]  # Arbitrary values for testing
-NUM_TOKENS = [1, 7, 83, 4096]  # Arbitrary values for testing
+                8193, 17920]  # Arbitrary values for testing
+NUM_TOKENS = [1, 7, 83, 4096, 131072]  # Arbitrary values for testing

The SEGV might be detected during this test, but it might not. If asan runs during this, it should be detected.

[tlrmchlsmth]

OK, enabling auto-merge to get the fix in -- makes sense that this would be difficult to test