Sanjoy Das via llvm-dev
2017-Jan-03 06:18 UTC
[llvm-dev] RFC: Allow readnone and readonly functions to throw exceptions
LLVM today does not clearly specify if a function specified to not write to memory (i.e. readonly or readnone) is allowed to throw exceptions. LangRef is ambiguous on this issue. The normative statement is "[readnone/readonly functions] cannot unwind exceptions by calling the C++ exception throwing methods" which does not decide an answer for non C++ languages. It used to say (h/t Daniel Berlin): "This means that it cannot unwind exceptions by calling the C++ exception throwing methods, but could use the unwind instruction.", but that bit of documentation died with the unwind instruction. I'd like to separate unwindability from memory effects, and officially change our stance to be "readonly / readnone functions are allowed to throw exceptions". Here are two supporting reasons: # `resume` is already modeled as readnone The easiest way to verify this is via FunctionAttrs; it infers the following function as readnone: define void @f() personality i8 42 { resume i32 0 } Modeling `resume` as `readnone` is defensible -- it is a control flow transfer instruction, not so different from `ret`. Moreover, it _cannot_ be modeled as having observable side effects or writes to memory (`resume` cannot send packets over the network or write to a global) because otherwise we'd be unable to inline @f into @g below: define void @f(i32 %x) personality i32 3 { resume i32 %x } define i32 @g(i32 %x) personality i32 3 { invoke void @f(i32 %x) to label %normal unwind label %unwind normal: ret i32 0 unwind: %t = landingpad i32 cleanup ret i32 %t } since it gets rid of a `resume` and thus a side effect (by assumption). # We're probably already there (but we need an audit) All said and done, the situation is not as "loosey goosey" as I made it sound like. mayHaveSideEffects() is defined as "mayWriteToMemory() || mayThrow()"; and this shows in e.g. EarlyCSE which will refuse to DCE the call to @f in @g declare void @f() readnone define void @g() { call void @f() ret void } unless @f is also marked nounwind. I've already fixed the one other instance I was aware of in https://reviews.llvm.org/rL290794 (but I will revert that patch if we decide against this RFC). We won't lose any expressive power either -- if there are situations where we have important optimizations firing under the "readonly implies nounwind" assumption, we can either - Teach FunctionAttrs to infer nounwind for readonly functions with C++ unwind personalities. - For external declarations generated by the compiler (say from the standard library): if the functions are actually nounwind, mark them as nounwind; and not rely on LLVM inferring nounwind from readonly. My (unrealistic?) hope is that this would mostly be a specification change and not involve a lot of code fixes. The change is also trivially upgrade-safe for older bitcode -- calls to readonly / readnone functions that do not throw _may_ get optimized less, but that should not be a correctness problem. What do you think? -- Sanjoy
Michael Kuperstein via llvm-dev
2017-Jan-03 07:49 UTC
[llvm-dev] RFC: Allow readnone and readonly functions to throw exceptions
This sounds right to me. IIUC, historically, readonly and readnone are meant to model the "pure" and "const" GCC attributes. These attributes make pretty strong guarantees: "[a pure] function can be subject to common subexpression elimination and loop optimization just as an arithmetic operator would be. These functions should be declared with the attribute pure [...] Interesting non-pure functions are functions with infinite loops or those depending on volatile memory or other system resource, that may change between two consecutive calls (such as feof in a multithreading environment)." In particular, pure/const imply termination - something that's not entirely clear w.r.t readonly. However, apparently, they don't imply nothrow. I've actually always thought they *do* imply it - and said so on-list :-) - but it looks like GCC itself doesn't interpret them that way. E.g. see John Regher's example here: https://t.co/REzy5m1tT3 So there's at least one use-case for possibly throwing readonly/readnone. As a side note, I'm slightly less optimistic about the amount of required code fixes. One thing that comes to mind is that we need to make sure we mark all(?) the intrinsics currently marked readonly/argmemonly/readnone as nothrow. This should be mostly mechanical, I hope, but it's a decent amount of churn. Michael On 2 January 2017 at 22:18, Sanjoy Das via llvm-dev <llvm-dev at lists.llvm.org> wrote:> LLVM today does not clearly specify if a function specified to not > write to memory (i.e. readonly or readnone) is allowed to throw > exceptions. > > LangRef is ambiguous on this issue. The normative statement is > "[readnone/readonly functions] cannot unwind exceptions by calling the > C++ exception throwing methods" which does not decide an answer for > non C++ languages. It used to say (h/t Daniel Berlin): "This means > that it cannot unwind exceptions by calling the C++ exception throwing > methods, but could use the unwind instruction.", but that bit of > documentation died with the unwind instruction. > > I'd like to separate unwindability from memory effects, and officially > change our stance to be "readonly / readnone functions are allowed to > throw exceptions". > > Here are two supporting reasons: > > # `resume` is already modeled as readnone > > The easiest way to verify this is via FunctionAttrs; it infers the > following function as readnone: > > define void @f() personality i8 42 { > resume i32 0 > } > > > Modeling `resume` as `readnone` is defensible -- it is a control flow > transfer instruction, not so different from `ret`. Moreover, it > _cannot_ be modeled as having observable side effects or writes to > memory (`resume` cannot send packets over the network or write to a > global) because otherwise we'd be unable to inline @f into @g below: > > define void @f(i32 %x) personality i32 3 { > resume i32 %x > } > > define i32 @g(i32 %x) personality i32 3 { > invoke void @f(i32 %x) to label %normal unwind label %unwind > > normal: > ret i32 0 > > unwind: > %t = landingpad i32 cleanup > ret i32 %t > } > > since it gets rid of a `resume` and thus a side effect (by > assumption). > > > # We're probably already there (but we need an audit) > > All said and done, the situation is not as "loosey goosey" as I made > it sound like. mayHaveSideEffects() is defined as "mayWriteToMemory() > || mayThrow()"; and this shows in e.g. EarlyCSE which will refuse to > DCE the call to @f in @g > > declare void @f() readnone > > define void @g() { > call void @f() > ret void > } > > unless @f is also marked nounwind. > > I've already fixed the one other instance I was aware of in > https://reviews.llvm.org/rL290794 (but I will revert that patch if we > decide against this RFC). > > We won't lose any expressive power either -- if there are situations > where we have important optimizations firing under the "readonly > implies nounwind" assumption, we can either > > - Teach FunctionAttrs to infer nounwind for readonly functions with > C++ unwind personalities. > > - For external declarations generated by the compiler (say from the > standard library): if the functions are actually nounwind, mark > them as nounwind; and not rely on LLVM inferring nounwind from > readonly. > > > My (unrealistic?) hope is that this would mostly be a specification > change and not involve a lot of code fixes. > > The change is also trivially upgrade-safe for older bitcode -- calls > to readonly / readnone functions that do not throw _may_ get optimized > less, but that should not be a correctness problem. > > What do you think? > > -- Sanjoy > _______________________________________________ > LLVM Developers mailing list > llvm-dev at lists.llvm.org > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev >-------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20170102/04a305f9/attachment.html>
Chandler Carruth via llvm-dev
2017-Jan-03 08:59 UTC
[llvm-dev] RFC: Allow readnone and readonly functions to throw exceptions
I like the direction of this RFC and agree with Michael's points about it. The "pure" and "const" history is definitely there, but I don't think it makes sense any more. I think narrow, precise, and well specified attributes are *much* better for LLVM's IR, especially as we diversify the set of frontends and language semantics we support. There will be plenty of code changes required, but I think the changes are tractable (these are relatively easy to audit for) and not risky. If Sanjoy has the cycles to run with this, fantastic. One thing we should make sure to do is update the langref to be *really clear* here. =] But I suspect Sanjoy is all over that. On Mon, Jan 2, 2017 at 11:49 PM Michael Kuperstein via llvm-dev < llvm-dev at lists.llvm.org> wrote:> This sounds right to me. > > IIUC, historically, readonly and readnone are meant to model the "pure" > and "const" GCC attributes. These attributes make pretty strong guarantees: > > "[a pure] function can be subject to common subexpression elimination and > loop optimization just as an arithmetic operator would be. These functions > should be declared with the attribute pure [...] Interesting non-pure > functions are functions with infinite loops or those depending on volatile > memory or other system resource, that may change between two consecutive > calls (such as feof in a multithreading environment)." > > In particular, pure/const imply termination - something that's not > entirely clear w.r.t readonly. However, apparently, they don't imply > nothrow. I've actually always thought they *do* imply it - and said so > on-list :-) - but it looks like GCC itself doesn't interpret them that way. > E.g. see John Regher's example here: https://t.co/REzy5m1tT3 > So there's at least one use-case for possibly throwing readonly/readnone. > > As a side note, I'm slightly less optimistic about the amount of required > code fixes. One thing that comes to mind is that we need to make sure we > mark all(?) the intrinsics currently marked readonly/argmemonly/readnone as > nothrow. This should be mostly mechanical, I hope, but it's a decent amount > of churn. > > Michael > > > > > On 2 January 2017 at 22:18, Sanjoy Das via llvm-dev < > llvm-dev at lists.llvm.org> wrote: > > LLVM today does not clearly specify if a function specified to not > write to memory (i.e. readonly or readnone) is allowed to throw > exceptions. > > LangRef is ambiguous on this issue. The normative statement is > "[readnone/readonly functions] cannot unwind exceptions by calling the > C++ exception throwing methods" which does not decide an answer for > non C++ languages. It used to say (h/t Daniel Berlin): "This means > that it cannot unwind exceptions by calling the C++ exception throwing > methods, but could use the unwind instruction.", but that bit of > documentation died with the unwind instruction. > > I'd like to separate unwindability from memory effects, and officially > change our stance to be "readonly / readnone functions are allowed to > throw exceptions". > > Here are two supporting reasons: > > # `resume` is already modeled as readnone > > The easiest way to verify this is via FunctionAttrs; it infers the > following function as readnone: > > define void @f() personality i8 42 { > resume i32 0 > } > > > Modeling `resume` as `readnone` is defensible -- it is a control flow > transfer instruction, not so different from `ret`. Moreover, it > _cannot_ be modeled as having observable side effects or writes to > memory (`resume` cannot send packets over the network or write to a > global) because otherwise we'd be unable to inline @f into @g below: > > define void @f(i32 %x) personality i32 3 { > resume i32 %x > } > > define i32 @g(i32 %x) personality i32 3 { > invoke void @f(i32 %x) to label %normal unwind label %unwind > > normal: > ret i32 0 > > unwind: > %t = landingpad i32 cleanup > ret i32 %t > } > > since it gets rid of a `resume` and thus a side effect (by > assumption). > > > # We're probably already there (but we need an audit) > > All said and done, the situation is not as "loosey goosey" as I made > it sound like. mayHaveSideEffects() is defined as "mayWriteToMemory() > || mayThrow()"; and this shows in e.g. EarlyCSE which will refuse to > DCE the call to @f in @g > > declare void @f() readnone > > define void @g() { > call void @f() > ret void > } > > unless @f is also marked nounwind. > > I've already fixed the one other instance I was aware of in > https://reviews.llvm.org/rL290794 (but I will revert that patch if we > decide against this RFC). > > We won't lose any expressive power either -- if there are situations > where we have important optimizations firing under the "readonly > implies nounwind" assumption, we can either > > - Teach FunctionAttrs to infer nounwind for readonly functions with > C++ unwind personalities. > > - For external declarations generated by the compiler (say from the > standard library): if the functions are actually nounwind, mark > them as nounwind; and not rely on LLVM inferring nounwind from > readonly. > > > My (unrealistic?) hope is that this would mostly be a specification > change and not involve a lot of code fixes. > > The change is also trivially upgrade-safe for older bitcode -- calls > to readonly / readnone functions that do not throw _may_ get optimized > less, but that should not be a correctness problem. > > What do you think? > > -- Sanjoy > _______________________________________________ > LLVM Developers mailing list > llvm-dev at lists.llvm.org > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev > > > _______________________________________________ > LLVM Developers mailing list > llvm-dev at lists.llvm.org > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev >-------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20170103/7dee56f8/attachment-0001.html>
Sanjoy Das via llvm-dev
2017-Jan-03 17:59 UTC
[llvm-dev] RFC: Allow readnone and readonly functions to throw exceptions
Hi Michael, On Mon, Jan 2, 2017 at 11:49 PM, Michael Kuperstein <michael.kuperstein at gmail.com> wrote:> This sounds right to me. > > IIUC, historically, readonly and readnone are meant to model the "pure" and > "const" GCC attributes. These attributes make pretty strong guarantees: > > "[a pure] function can be subject to common subexpression elimination and > loop optimization just as an arithmetic operator would be. These functions > should be declared with the attribute pure [...] Interesting non-pure > functions are functions with infinite loops or those depending on volatile > memory or other system resource, that may change between two consecutive > calls (such as feof in a multithreading environment)." > > In particular, pure/const imply termination - something that's not entirely > clear w.r.t readonly. However, apparently, they don't imply nothrow. I've > actually always thought they *do* imply it - and said so on-list :-) - but > it looks like GCC itself doesn't interpret them that way. E.g. see John > Regher's example here: https://t.co/REzy5m1tT3 > So there's at least one use-case for possibly throwing readonly/readnone.One important thing to note then: clang marks const and pure functions as nounwind *explicitly*. That needs to be fixed. https://godbolt.org/g/SMF4C9> As a side note, I'm slightly less optimistic about the amount of required > code fixes. One thing that comes to mind is that we need to make sure we > mark all(?) the intrinsics currently marked readonly/argmemonly/readnone as > nothrow. This should be mostly mechanical, I hope, but it's a decent amount > of churn.The behavior around intrinsics today is that they're implicitly marked NoUnwind unless they're specifically annotated as [Throws], of which there are very few instances (statepoints, stackmap, patchpoint, coro_*). My intent was to (document and) keep this behavior. -- Sanjoy
Reid Kleckner via llvm-dev
2017-Jan-03 19:27 UTC
[llvm-dev] RFC: Allow readnone and readonly functions to throw exceptions
I've always been in the "readnone implies nounwind" camp, but I've changed my mind, and I'm in favor of this RFC now. I think in the past I was hung up on the idea that every language typically has some "exception object" that is stored in memory, and the user code interrogates it by loading, so we need to model stores when throwing an exception. Right now I'm imagining an EH personality that uses i32 error codes as its exceptional return value, which would look like this: invoke void @f() to label %next unwind label %lpad ; could be readnone ... lpad: %vals = i32 landingpad ... ; suppose there is no selector value in this personality define void @f() readnone { resume i32 42 ; imagine that resume could initiate exceptional control flow like the old 'unwind' instruction } With a personality like this, there are no llvm-visible memory operations looking at the exception, so it makes sense for @f to be readnone. On Mon, Jan 2, 2017 at 10:18 PM, Sanjoy Das via llvm-dev < llvm-dev at lists.llvm.org> wrote:> LLVM today does not clearly specify if a function specified to not > write to memory (i.e. readonly or readnone) is allowed to throw > exceptions. > > LangRef is ambiguous on this issue. The normative statement is > "[readnone/readonly functions] cannot unwind exceptions by calling the > C++ exception throwing methods" which does not decide an answer for > non C++ languages. It used to say (h/t Daniel Berlin): "This means > that it cannot unwind exceptions by calling the C++ exception throwing > methods, but could use the unwind instruction.", but that bit of > documentation died with the unwind instruction. > > I'd like to separate unwindability from memory effects, and officially > change our stance to be "readonly / readnone functions are allowed to > throw exceptions". > > Here are two supporting reasons: > > # `resume` is already modeled as readnone > > The easiest way to verify this is via FunctionAttrs; it infers the > following function as readnone: > > define void @f() personality i8 42 { > resume i32 0 > } > > > Modeling `resume` as `readnone` is defensible -- it is a control flow > transfer instruction, not so different from `ret`. Moreover, it > _cannot_ be modeled as having observable side effects or writes to > memory (`resume` cannot send packets over the network or write to a > global) because otherwise we'd be unable to inline @f into @g below: > > define void @f(i32 %x) personality i32 3 { > resume i32 %x > } > > define i32 @g(i32 %x) personality i32 3 { > invoke void @f(i32 %x) to label %normal unwind label %unwind > > normal: > ret i32 0 > > unwind: > %t = landingpad i32 cleanup > ret i32 %t > } > > since it gets rid of a `resume` and thus a side effect (by > assumption). > > > # We're probably already there (but we need an audit) > > All said and done, the situation is not as "loosey goosey" as I made > it sound like. mayHaveSideEffects() is defined as "mayWriteToMemory() > || mayThrow()"; and this shows in e.g. EarlyCSE which will refuse to > DCE the call to @f in @g > > declare void @f() readnone > > define void @g() { > call void @f() > ret void > } > > unless @f is also marked nounwind. > > I've already fixed the one other instance I was aware of in > https://reviews.llvm.org/rL290794 (but I will revert that patch if we > decide against this RFC). > > We won't lose any expressive power either -- if there are situations > where we have important optimizations firing under the "readonly > implies nounwind" assumption, we can either > > - Teach FunctionAttrs to infer nounwind for readonly functions with > C++ unwind personalities. > > - For external declarations generated by the compiler (say from the > standard library): if the functions are actually nounwind, mark > them as nounwind; and not rely on LLVM inferring nounwind from > readonly. > > > My (unrealistic?) hope is that this would mostly be a specification > change and not involve a lot of code fixes. > > The change is also trivially upgrade-safe for older bitcode -- calls > to readonly / readnone functions that do not throw _may_ get optimized > less, but that should not be a correctness problem. > > What do you think? > > -- Sanjoy > _______________________________________________ > LLVM Developers mailing list > llvm-dev at lists.llvm.org > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev >-------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20170103/230d4a56/attachment-0001.html>
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