rustc_trans: do not generate allocas for unused locals. #35916
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eddyb
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rustc_trans: do not generate allocas for unused locals. This fixes a regression observed in a [`mio` test](https://travis-ci.org/carllerche/mio/jobs/152142886) which was referencing a 4MB `const` array. Even though MIR rvalue promotion would promote the borrow of the array, a dead temp was left behind. As the array doesn't have an immediate type, an `alloca` was generated for it, even though it had no uses. The fix is pretty dumb: assume that locals need to be borrowed or assigned before being used. And if it can't be used, it doesn't get an `alloca`, even if the type would otherwise demand it. This could change in the future, but all the MIR we generate now doesn't break that rule.
This was referenced Aug 23, 2016
eddyb
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rustc_trans: do not generate allocas for unused locals. This fixes a regression observed in a [`mio` test](https://travis-ci.org/carllerche/mio/jobs/152142886) which was referencing a 4MB `const` array. Even though MIR rvalue promotion would promote the borrow of the array, a dead temp was left behind. As the array doesn't have an immediate type, an `alloca` was generated for it, even though it had no uses. The fix is pretty dumb: assume that locals need to be borrowed or assigned before being used. And if it can't be used, it doesn't get an `alloca`, even if the type would otherwise demand it. This could change in the future, but all the MIR we generate now doesn't break that rule.
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Aug 25, 2016
rustc_trans: do not generate allocas for unused locals. This fixes a regression observed in a [`mio` test](https://travis-ci.org/carllerche/mio/jobs/152142886) which was referencing a 4MB `const` array. Even though MIR rvalue promotion would promote the borrow of the array, a dead temp was left behind. As the array doesn't have an immediate type, an `alloca` was generated for it, even though it had no uses. The fix is pretty dumb: assume that locals need to be borrowed or assigned before being used. And if it can't be used, it doesn't get an `alloca`, even if the type would otherwise demand it. This could change in the future, but all the MIR we generate now doesn't break that rule.
Manishearth
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Aug 25, 2016
rustc_trans: do not generate allocas for unused locals. This fixes a regression observed in a [`mio` test](https://travis-ci.org/carllerche/mio/jobs/152142886) which was referencing a 4MB `const` array. Even though MIR rvalue promotion would promote the borrow of the array, a dead temp was left behind. As the array doesn't have an immediate type, an `alloca` was generated for it, even though it had no uses. The fix is pretty dumb: assume that locals need to be borrowed or assigned before being used. And if it can't be used, it doesn't get an `alloca`, even if the type would otherwise demand it. This could change in the future, but all the MIR we generate now doesn't break that rule.
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A way to remove otherwise unused locals from MIR There is a certain amount of desire for a pass which cleans up the provably unused variables (no assignments or reads). There has been an implementation of such pass by @scottcarr, and another (two!) implementations by me in my own dataflow efforts. PR like #35916 proves that this pass is useful even on its own, which is why I cherry-picked it out from my dataflow effort. @nikomatsakis previously expressed concerns over this pass not seeming to be very cheap to run and therefore unsuitable for regular cleanup duties. Turns out, regular cleanup of local declarations is not at all necessary, at least now, because majority of passes simply do not (or should not) care about them. That’s why it is viable to only run this pass once (perhaps a few more times in the future?) per function, right before translation. r? @eddyb or @nikomatsakis
bors
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Nov 3, 2016
A way to remove otherwise unused locals from MIR There is a certain amount of desire for a pass which cleans up the provably unused variables (no assignments or reads). There has been an implementation of such pass by @scottcarr, and another (two!) implementations by me in my own dataflow efforts. PR like #35916 proves that this pass is useful even on its own, which is why I cherry-picked it out from my dataflow effort. @nikomatsakis previously expressed concerns over this pass not seeming to be very cheap to run and therefore unsuitable for regular cleanup duties. Turns out, regular cleanup of local declarations is not at all necessary, at least now, because majority of passes simply do not (or should not) care about them. That’s why it is viable to only run this pass once (perhaps a few more times in the future?) per function, right before translation. r? @eddyb or @nikomatsakis
bors
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Nov 4, 2016
A way to remove otherwise unused locals from MIR There is a certain amount of desire for a pass which cleans up the provably unused variables (no assignments or reads). There has been an implementation of such pass by @scottcarr, and another (two!) implementations by me in my own dataflow efforts. PR like #35916 proves that this pass is useful even on its own, which is why I cherry-picked it out from my dataflow effort. @nikomatsakis previously expressed concerns over this pass not seeming to be very cheap to run and therefore unsuitable for regular cleanup duties. Turns out, regular cleanup of local declarations is not at all necessary, at least now, because majority of passes simply do not (or should not) care about them. That’s why it is viable to only run this pass once (perhaps a few more times in the future?) per function, right before translation. r? @eddyb or @nikomatsakis
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This fixes a regression observed in a
miotest which was referencing a 4MBconstarray.Even though MIR rvalue promotion would promote the borrow of the array, a dead temp was left behind.
As the array doesn't have an immediate type, an
allocawas generated for it, even though it had no uses.The fix is pretty dumb: assume that locals need to be borrowed or assigned before being used.
And if it can't be used, it doesn't get an
alloca, even if the type would otherwise demand it.This could change in the future, but all the MIR we generate now doesn't break that rule.