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Fix a warning for 64-bit 2D Morton codes #640

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merged 1 commit into from
Feb 23, 2022
Merged

Fix a warning for 64-bit 2D Morton codes #640

merged 1 commit into from
Feb 23, 2022

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aprokop
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@aprokop aprokop commented Feb 22, 2022

Introduced in #635.

We do (1 << 31). The problem is that MAX_INT (which is 2147483647) is one smaller than that, which leads to overflow.

NVCC correctly warns about that. But we don't catch Nvidia warnings in CI (see #272), so we missed this.
Not sure why none of the host compilers produced a warning.

This is the only place where it is necessary, all other places shift by less than 31.

This bug potentially leads to construction of a wrong hierarchy.

@aprokop aprokop added the bug Something isn't working label Feb 22, 2022
@aprokop aprokop mentioned this pull request Feb 22, 2022
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dalg24
dalg24 reviewed Feb 23, 2022
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Can you think of a test that would catch that issue?

src/details/ArborX_DetailsMortonCode.hpp Show resolved Hide resolved
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aprokop commented Feb 23, 2022

Can you think of a test that would catch that issue?

So far, no. In fact, unsigned int N = 1 << 31; and unsigned int N = 1u << 31; produce the same value on CPU. However, I had a severe performance degradation when running 2D hacked code on a real problem. I think it may only manifest on a GPU.

@aprokop aprokop removed the bug Something isn't working label Feb 23, 2022
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aprokop commented Feb 23, 2022 •
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OK, after careful examination, this is not a bug. It works correctly despite doing a weird thing. The slowdown I observed on GPU was actually caused by a different issue. So this PR is now solely to fix the warning.

@aprokop
Fix a warning "integer conversion resulted in a change of sign"
3f043de 
We do (1 << 31). The problem is that MAX_INT (which is 2147483647) is
smaller than that, which leads to overflow.

Despite that, `unsigned int N = 1 << 31;` produces correct result, same
as `unsigned int N = 1u << 31;`.
@aprokop aprokop changed the title Fix a bug for 64-bit 2D Morton codes Fix a warning for 64-bit 2D Morton codes Feb 23, 2022
@aprokop aprokop added the build Build and installation label Feb 23, 2022
masterleinad
masterleinad approved these changes Feb 23, 2022
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I think this is much better even if it's not a bug. The bit shift here really is understood as multiplication that should not overflow.

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aprokop commented Feb 23, 2022 •
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Builds passed, only warnings from the SYCL container build.

@aprokop aprokop merged commit f8d122e into arborx:master Feb 23, 2022
@aprokop aprokop deleted the bugfix branch February 23, 2022 20:34
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