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Posted to commits@nuttx.apache.org by GitBox <gi...@apache.org> on 2021/12/30 17:18:23 UTC
[GitHub] [incubator-nuttx] pkarashchenko opened a new pull request #5125: libc/math: fix log and logf calculations on ARMv7 (and maybe others)
pkarashchenko opened a new pull request #5125:
URL: https://github.com/apache/incubator-nuttx/pull/5125
## Summary
Probably this is a bug of a GCC, but on AMRv7 the code `if (relax_factor > 1)` generates `bne.n` instruction in `relax_factor` is `int`. Few lines above `relax_factor *= LOG_RELAX_MULTIPLIER;` is done without overflow check hence at some moment overflow occurs and `relax_factor` becomes a zero and condition `if (relax_factor > 1)` becomes always evaluated to `true` hence `epsilon` becomes zero always and `log` call never returns.
Probably this is not the best way to fix the bug (The best way is to report it to GCC), but this change allows to get correct behavior of `log` and `logf` for ARMv7 based MCUs
## Impact
Native math lib users
## Testing
Tested on SAME70 based device.
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[GitHub] [incubator-nuttx] pkarashchenko commented on pull request #5125: libc/math: fix log and logf calculations on ARMv7 (and maybe others)
Posted by GitBox <gi...@apache.org>.
pkarashchenko commented on pull request #5125:
URL: https://github.com/apache/incubator-nuttx/pull/5125#issuecomment-1003199336
Compiler folks recommend one of the possible solutions:
1. Use `-fwrapv` flag
2. Switch from `int` to `unsigned int`
3. Use builtin functions for overflow detection
Probably switching to `unsigned int` is the most lightweight option. I will redo the change
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[GitHub] [incubator-nuttx] pkarashchenko commented on pull request #5125: libc/math: fix log and logf calculations on ARMv7 (and maybe others)
Posted by GitBox <gi...@apache.org>.
pkarashchenko commented on pull request #5125:
URL: https://github.com/apache/incubator-nuttx/pull/5125#issuecomment-1003195261
Created https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103870 to track related issue on GCC side
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[GitHub] [incubator-nuttx] pkarashchenko edited a comment on pull request #5125: libc/math: fix log and logf calculations on ARMv7 (and maybe others)
Posted by GitBox <gi...@apache.org>.
pkarashchenko edited a comment on pull request #5125:
URL: https://github.com/apache/incubator-nuttx/pull/5125#issuecomment-1003199336
Compiler folks recommend one of the possible solutions:
1. Use `-fwrapv` flag
2. Switch from `int` to `unsigned int`
3. Use builtin functions for overflow detection
Probably switching to `unsigned int` is the most lightweight option, but it still require some overflow protection code.
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[GitHub] [incubator-nuttx] xiaoxiang781216 merged pull request #5125: libc/math: fix log and logf calculations on ARMv7 (and maybe others)
Posted by GitBox <gi...@apache.org>.
xiaoxiang781216 merged pull request #5125:
URL: https://github.com/apache/incubator-nuttx/pull/5125
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[GitHub] [incubator-nuttx] pkarashchenko commented on pull request #5125: libc/math: fix log and logf calculations on ARMv7 (and maybe others)
Posted by GitBox <gi...@apache.org>.
pkarashchenko commented on pull request #5125:
URL: https://github.com/apache/incubator-nuttx/pull/5125#issuecomment-1003120927
@patacongo I'm not sure who is the author of math lib code. Will appreciate your input on this change
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[GitHub] [incubator-nuttx] pkarashchenko edited a comment on pull request #5125: libc/math: fix log and logf calculations on ARMv7 (and maybe others)
Posted by GitBox <gi...@apache.org>.
pkarashchenko edited a comment on pull request #5125:
URL: https://github.com/apache/incubator-nuttx/pull/5125#issuecomment-1003199336
Compiler folks recommend one of the possible solutions:
1. Use `-fwrapv` flag
2. Switch from `int` to `unsigned int`
3. Use builtin functions for overflow detection
Probably switching to `unsigned int` is the most lightweight option, but it still require some overflow protection code.
I can redo the change to switch to `unsigned int` + overflow check that will be more lightweight for architectures that use `softfp`. However proposed change seems to be more robust and reliable even if it consumes some more cycles. I will wait for some additional feedback in comments
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