Slightly optimise GCC static destruction #11727
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Static destruction code may still end up in a GCC image despite our efforts. Use of `SingletonPtr` will eliminate most, but references can still occur. For IAR and ARM toolchains they can be totally eliminated in the compiler (see ARMmbed#11726 for ARM). As the Clang attributes have been submitted for C++ standardisation (http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p1247r0.html), it seems likely that one day GCC will also support this, so it seems not worthwhile to create a `NotDestroyed<T>` template wrapper as a workaround, as I was originally intending. In the meantime, GCC is inserting calls to `__eabi_atexit`. That is stubbed out, so whatever it attempts to register will not be called. We can't stop it putting the calls in, but we can transparently slightly reduce code size by telling it to use `atexit` instead. It needs to pass fewer parameters for us to ignore, so the callsites become a little smaller.
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If we test this flag with some examples, how much size difference does it make? |
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Just tried it with a large (cloud client) image rather than my small synthetic examples, and found it made that bigger :) There's obviously a secondary effect here - maybe something to do with failure to exclude the ".fini" array. Will dig into it. Even if this does work properly, saving is going to be pretty minor, compared to #11726. |
0xc0170
approved these changes
Oct 22, 2019
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Okay, can't pin down a particular reason for the increases here. If it's not a straightforward (small) win, I'll skip it - not worth spending time on. |
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Description
Static destruction code may still end up in a GCC image despite our efforts. Use of
SingletonPtrwill eliminate most, but references can still occur.For IAR and ARM toolchains they can be totally eliminated in the compiler (see #11726 for ARM). As the Clang attributes have been submitted for C++ standardisation (http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p1247r0.html), it seems likely that one day GCC will also support this, so it seems not worthwhile to create a
NotDestroyed<T>template wrapper as a workaround, as I was originally intending.In the meantime, GCC is inserting calls to
__eabi_atexit. That is stubbed out, so whatever it attempts to register will not be called. We can't stop it putting the calls in, but we can transparently slightlyreduce code size by telling it to use
atexitinstead. It needs to pass fewer parameters for us to ignore, so the callsites become a little smaller.Pull request type
Reviewers
@pan-, @bulislaw, @0xc0170