Sanjoy Das via llvm-dev
2016-Feb-24 23:57 UTC
[llvm-dev] Possible soundness issue with available_externally (split from "RFC: Add guard intrinsics")
Hi all, This is something that came up in the "RFC: Add guard intrinsics to LLVM" thread; and while I'm not exactly blocked on this, figuring out a path forward here will be helpful in deciding if we can use the available_externally linkage type to expression certain semantic properties guard intrinsics will have. Let's start with an example that shows that we have a problem (direct copy/paste from the guard intrinsics thread). Say we have: ``` void foo() available_externally { %t0 = load atomic %ptr %t1 = load atomic %ptr if (%t0 != %t1) print("X"); } void main() { foo(); print("Y"); } ``` The possible behaviors of the above program are {print("X"), print("Y")} or {print("Y")}. But if we run opt then we have ``` void foo() available_externally readnone nounwind { ;; After CSE'ing the two loads and folding the condition } void main() { foo(); print("Y"); } ``` and some generic reordering ``` void foo() available_externally readnone nounwind { ;; After CSE'ing the two loads and folding the condition } void main() { print("Y"); foo(); // legal since we're moving a readnone nounwind function that // was guaranteed to execute (hence can't have UB) } ``` If we do not inline @foo(), and instead re-link the call site in @main to some non-optimized copy (or differently optimized copy) of @foo, then it is possible for the program to have the behavior {print("Y"); print ("X")}, which was disallowed in the earlier program. In other words, opt refined the semantics of @foo() (i.e. reduced the set of behaviors it may have) in ways that would make later optimizations invalid if we de-refine the implementation of @foo(). The above example is clearly fabricated, but such cases can come up even if everything is optimized to the same level. E.g. one of the atomic loads in the unrefined implementation of @foo() could have been hidden behind a function call, whose body existed in only one module. That module would then be able to refine @foo() to `ret void` but other modules won't. The only solution I can think of is to redefine available_externally to mean "the only kind of IPO/IPA you can do over a call to this function is to inline it". Redefining available_externally this way will also let us soundly use it to represent calls to functions that have guard intrinsics, since a failed guard intrinsic basically replaces the function with a "very de-refined" implementation (the interpreter). What do you think? I don't think implementing the above above will be very difficult, but needless to say, it will still be a fairly non-trivial semantic change (hence I'm not directly jumping to implementation). -- Sanjoy
Duncan P. N. Exon Smith via llvm-dev
2016-Feb-25 02:51 UTC
[llvm-dev] Possible soundness issue with available_externally (split from "RFC: Add guard intrinsics")
> On 2016-Feb-24, at 15:57, Sanjoy Das via llvm-dev <llvm-dev at lists.llvm.org> wrote: > > Hi all, > > This is something that came up in the "RFC: Add guard intrinsics to > LLVM" thread; and while I'm not exactly blocked on this, figuring out > a path forward here will be helpful in deciding if we can use the > available_externally linkage type to expression certain semantic > properties guard intrinsics will have. > > Let's start with an example that shows that we have a problem (direct > copy/paste from the guard intrinsics thread). Say we have: > > ``` > void foo() available_externally { > %t0 = load atomic %ptr > %t1 = load atomic %ptr > if (%t0 != %t1) print("X"); > } > void main() { > foo(); > print("Y"); > } > ``` > > The possible behaviors of the above program are {print("X"), > print("Y")} or {print("Y")}. But if we run opt then we have > > ``` > void foo() available_externally readnone nounwind { > ;; After CSE'ing the two loads and folding the condition > } > void main() { > foo(); > print("Y"); > } > ``` > > and some generic reordering > > ``` > void foo() available_externally readnone nounwind { > ;; After CSE'ing the two loads and folding the condition > } > void main() { > print("Y"); > foo(); // legal since we're moving a readnone nounwind function that > // was guaranteed to execute (hence can't have UB) > } > ``` > > If we do not inline @foo(), and instead re-link the call site in @main > to some non-optimized copy (or differently optimized copy) of @foo, > then it is possible for the program to have the behavior {print("Y"); > print ("X")}, which was disallowed in the earlier program. > > In other words, opt refined the semantics of @foo() (i.e. reduced the > set of behaviors it may have) in ways that would make later > optimizations invalid if we de-refine the implementation of @foo().I'm probably missing something obvious here. How could the result of `%t0 != %t1` be different at optimization time in one file than from runtime in the "real" implementation? Doesn't this make the CSE invalid? Does linkonce_odr linkage have the same problem? - If so, do you want to change it too? - Else, why not?> The above example is clearly fabricated, but such cases can come up > even if everything is optimized to the same level. E.g. one of the > atomic loads in the unrefined implementation of @foo() could have been > hidden behind a function call, whose body existed in only one module. > That module would then be able to refine @foo() to `ret void` but > other modules won't. > > The only solution I can think of is to redefine available_externally > to mean "the only kind of IPO/IPA you can do over a call to this > function is to inline it". Redefining available_externally this way > will also let us soundly use it to represent calls to functions that > have guard intrinsics, since a failed guard intrinsic basically > replaces the function with a "very de-refined" implementation (the > interpreter). > > What do you think? I don't think implementing the above above will be > very difficult, but needless to say, it will still be a fairly > non-trivial semantic change (hence I'm not directly jumping to > implementation).This linkage is used in three places (that I know of) by clang: 1. C-style `inline` functions. 2. Functions defined in C++ template classes with external explicit instantiations, e.g. S::foo() in: template <class T> struct S { void foo() {} }; void bar() { S<int>().foo(); } extern template struct S<int>; 3. -flto=thin cross-module function importing. (No comment on (1); its exact semantics are a little fuzzy to me.) For (2) and (3), the current behaviour seems correct, and I'd be hesitant to lose optimizing power. (2) is under the "ODR" rule, and I think we've been applying the same logic to (3). Unless, are you saying ODR isn't enough? Assuming you need this new definition (but under ODR, the semantics are correct), I would rather split the linkage than change it. E.g., use a name like available_externally_odr for (2) and (3).
Sanjoy Das via llvm-dev
2016-Feb-25 03:09 UTC
[llvm-dev] Possible soundness issue with available_externally (split from "RFC: Add guard intrinsics")
On Wed, Feb 24, 2016 at 6:51 PM, Duncan P. N. Exon Smith <dexonsmith at apple.com> wrote:>> If we do not inline @foo(), and instead re-link the call site in @main >> to some non-optimized copy (or differently optimized copy) of @foo, >> then it is possible for the program to have the behavior {print("Y"); >> print ("X")}, which was disallowed in the earlier program. >> >> In other words, opt refined the semantics of @foo() (i.e. reduced the >> set of behaviors it may have) in ways that would make later >> optimizations invalid if we de-refine the implementation of @foo(). > > I'm probably missing something obvious here. How could the result of > `%t0 != %t1` be different at optimization time in one file than from > runtime in the "real" implementation? Doesn't this make the CSE > invalid?`%t0` and `%t1` are "allowed" to "always be the same", i.e. an implementation of @foo that always feeds in the same value for `%t0` and `%t1` is a valid implementation (which is why the CSE was valid); but it is not the *only* valid implementation. If I don't CSE the two load instructions (also a valid thing to do), and this is a second thread writing to `%par`, then the two values loaded can be different, and you could end up printing `"X"` in `@foo`. Did that make sense?> Does linkonce_odr linkage have the same problem? > - If so, do you want to change it too? > - Else, why not?Going by the specification in the LangRef, I'd say it depends on how you define "definitive". If you're allowed to replace the body of a function with a differently optimized body, then the above problem exists.>> The above example is clearly fabricated, but such cases can come up >> even if everything is optimized to the same level. E.g. one of the >> atomic loads in the unrefined implementation of @foo() could have been >> hidden behind a function call, whose body existed in only one module. >> That module would then be able to refine @foo() to `ret void` but >> other modules won't. >> >> The only solution I can think of is to redefine available_externally >> to mean "the only kind of IPO/IPA you can do over a call to this >> function is to inline it". Redefining available_externally this way >> will also let us soundly use it to represent calls to functions that >> have guard intrinsics, since a failed guard intrinsic basically >> replaces the function with a "very de-refined" implementation (the >> interpreter). >> >> What do you think? I don't think implementing the above above will be >> very difficult, but needless to say, it will still be a fairly >> non-trivial semantic change (hence I'm not directly jumping to >> implementation). > > This linkage is used in three places (that I know of) by clang: > > 1. C-style `inline` functions. > 2. Functions defined in C++ template classes with external explicit > instantiations, e.g. S::foo() in: > > template <class T> struct S { void foo() {} }; > void bar() { S<int>().foo(); } > extern template struct S<int>; > > 3. -flto=thin cross-module function importing. > > (No comment on (1); its exact semantics are a little fuzzy to me.) > For (2) and (3), the current behaviour seems correct, and I'd be > hesitant to lose optimizing power. (2) is under the "ODR" rule, and > I think we've been applying the same logic to (3). Unless, are you > saying ODR isn't enough?By ODR, do you mean you only have one definition of the function in the whole link (i.e. across all modules you'll link together)? Then yes, ODR should be enough to avoid this. But in any place where the linker sees two differently optimized definitions for a function and picks one as the definitive version all non-inlined calls link to, we have this problem.> Assuming you need this new definition (but under ODR, the semantics > are correct), I would rather split the linkage than change it. E.g., > use a name like available_externally_odr for (2) and (3).If what I said above is correct (i.e. ODR == OD across everything you're linking into your final executable) then splitting the linkage / adding a new one is probably the best alternative. -- Sanjoy
Xinliang David Li via llvm-dev
2016-Feb-25 06:23 UTC
[llvm-dev] Possible soundness issue with available_externally (split from "RFC: Add guard intrinsics")
We have seen similar issues with COMDAT in our production environment -- basically we can not safely mix object files compiled with different -mxxx options. One scenario is that a users puts target specific (multi-versioned) functions in one file and build it with option such as -mavx. Such functions are called with runtime guard so there is no issue. However comdat functions brought in from common headers can be problematic -- as if the avx version of the function gets picked by the linker, the program will crash when running on hardware without AVX. One proposal to avoid this is to do function privatization. thanks, David On Wed, Feb 24, 2016 at 3:57 PM, Sanjoy Das via llvm-dev < llvm-dev at lists.llvm.org> wrote:> Hi all, > > This is something that came up in the "RFC: Add guard intrinsics to > LLVM" thread; and while I'm not exactly blocked on this, figuring out > a path forward here will be helpful in deciding if we can use the > available_externally linkage type to expression certain semantic > properties guard intrinsics will have. > > Let's start with an example that shows that we have a problem (direct > copy/paste from the guard intrinsics thread). Say we have: > > ``` > void foo() available_externally { > %t0 = load atomic %ptr > %t1 = load atomic %ptr > if (%t0 != %t1) print("X"); > } > void main() { > foo(); > print("Y"); > } > ``` > > The possible behaviors of the above program are {print("X"), > print("Y")} or {print("Y")}. But if we run opt then we have > > ``` > void foo() available_externally readnone nounwind { > ;; After CSE'ing the two loads and folding the condition > } > void main() { > foo(); > print("Y"); > } > ``` > > and some generic reordering > > ``` > void foo() available_externally readnone nounwind { > ;; After CSE'ing the two loads and folding the condition > } > void main() { > print("Y"); > foo(); // legal since we're moving a readnone nounwind function that > // was guaranteed to execute (hence can't have UB) > } > ``` > > If we do not inline @foo(), and instead re-link the call site in @main > to some non-optimized copy (or differently optimized copy) of @foo, > then it is possible for the program to have the behavior {print("Y"); > print ("X")}, which was disallowed in the earlier program. > > In other words, opt refined the semantics of @foo() (i.e. reduced the > set of behaviors it may have) in ways that would make later > optimizations invalid if we de-refine the implementation of @foo(). > > The above example is clearly fabricated, but such cases can come up > even if everything is optimized to the same level. E.g. one of the > atomic loads in the unrefined implementation of @foo() could have been > hidden behind a function call, whose body existed in only one module. > That module would then be able to refine @foo() to `ret void` but > other modules won't. > > The only solution I can think of is to redefine available_externally > to mean "the only kind of IPO/IPA you can do over a call to this > function is to inline it". Redefining available_externally this way > will also let us soundly use it to represent calls to functions that > have guard intrinsics, since a failed guard intrinsic basically > replaces the function with a "very de-refined" implementation (the > interpreter). > > What do you think? I don't think implementing the above above will be > very difficult, but needless to say, it will still be a fairly > non-trivial semantic change (hence I'm not directly jumping to > implementation). > > > -- 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/20160224/bc480c45/attachment.html>
James Y Knight via llvm-dev
2016-Feb-25 19:41 UTC
[llvm-dev] Possible soundness issue with available_externally (split from "RFC: Add guard intrinsics")
While we're talking about this, I'd just mention again that the same issue arises for *normal* functions too, when linked into a shared library: int foo() { return 1; } int bar() { return foo(); } Now, compare: clang -fPIC -O1 -S -o - test.c gcc -fPIC -O1 -S -o - test.c GCC will refuse to inline foo into bar, or use any information about foo in compiling bar, because foo is exported in the dynamic symbol table, and thus replaceable via symbol interposition. Clang assumes that you won't do that, or that you don't care what happens if you do. It will happily inline. And, in absense of inlining (e.g. if foo is too long to inline), clang will deduce function attributes about foo and rely on those in bar -- despite that the call goes through the PLT and could in fact be an entirely different unrelated implementation (or, for that matter, a differently-optimized version of the same implementation). Is that *really* okay? On Wed, Feb 24, 2016 at 6:57 PM, Sanjoy Das via llvm-dev < llvm-dev at lists.llvm.org> wrote:> Hi all, > > This is something that came up in the "RFC: Add guard intrinsics to > LLVM" thread; and while I'm not exactly blocked on this, figuring out > a path forward here will be helpful in deciding if we can use the > available_externally linkage type to expression certain semantic > properties guard intrinsics will have. > > Let's start with an example that shows that we have a problem (direct > copy/paste from the guard intrinsics thread). Say we have: > > ``` > void foo() available_externally { > %t0 = load atomic %ptr > %t1 = load atomic %ptr > if (%t0 != %t1) print("X"); > } > void main() { > foo(); > print("Y"); > } > ``` > > The possible behaviors of the above program are {print("X"), > print("Y")} or {print("Y")}. But if we run opt then we have > > ``` > void foo() available_externally readnone nounwind { > ;; After CSE'ing the two loads and folding the condition > } > void main() { > foo(); > print("Y"); > } > ``` > > and some generic reordering > > ``` > void foo() available_externally readnone nounwind { > ;; After CSE'ing the two loads and folding the condition > } > void main() { > print("Y"); > foo(); // legal since we're moving a readnone nounwind function that > // was guaranteed to execute (hence can't have UB) > } > ``` > > If we do not inline @foo(), and instead re-link the call site in @main > to some non-optimized copy (or differently optimized copy) of @foo, > then it is possible for the program to have the behavior {print("Y"); > print ("X")}, which was disallowed in the earlier program. > > In other words, opt refined the semantics of @foo() (i.e. reduced the > set of behaviors it may have) in ways that would make later > optimizations invalid if we de-refine the implementation of @foo(). > > The above example is clearly fabricated, but such cases can come up > even if everything is optimized to the same level. E.g. one of the > atomic loads in the unrefined implementation of @foo() could have been > hidden behind a function call, whose body existed in only one module. > That module would then be able to refine @foo() to `ret void` but > other modules won't. > > The only solution I can think of is to redefine available_externally > to mean "the only kind of IPO/IPA you can do over a call to this > function is to inline it". Redefining available_externally this way > will also let us soundly use it to represent calls to functions that > have guard intrinsics, since a failed guard intrinsic basically > replaces the function with a "very de-refined" implementation (the > interpreter). > > What do you think? I don't think implementing the above above will be > very difficult, but needless to say, it will still be a fairly > non-trivial semantic change (hence I'm not directly jumping to > implementation). > > > -- 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/20160225/f4b0f420/attachment.html>
Hal Finkel via llvm-dev
2016-Feb-27 01:50 UTC
[llvm-dev] Possible soundness issue with available_externally (split from "RFC: Add guard intrinsics")
----- Original Message -----> From: "James Y Knight via llvm-dev" <llvm-dev at lists.llvm.org> > To: "Sanjoy Das" <sanjoy at playingwithpointers.com> > Cc: "llvm-dev" <llvm-dev at lists.llvm.org> > Sent: Thursday, February 25, 2016 1:41:43 PM > Subject: Re: [llvm-dev] Possible soundness issue with > available_externally (split from "RFC: Add guard intrinsics")> While we're talking about this, I'd just mention again that the same > issue arises for *normal* functions too, when linked into a shared > library: > int foo() { return 1; }> int bar() { return foo(); }> Now, compare: > clang -fPIC -O1 -S -o - test.c> gcc -fPIC -O1 -S -o - test.c> GCC will refuse to inline foo into bar, or use any information about > foo in compiling bar, because foo is exported in the dynamic symbol > table, and thus replaceable via symbol interposition.> Clang assumes that you won't do that, or that you don't care what > happens if you do. It will happily inline. And, in absense of > inlining (e.g. if foo is too long to inline), clang will deduce > function attributes about foo and rely on those in bar -- despite > that the call goes through the PLT and could in fact be an entirely > different unrelated implementation (or, for that matter, a > differently-optimized version of the same implementation).> Is that *really* okay?I'm comfortable with saying that symbol interposition falls outside of the model we have for the targeted system (at least by default), and thus, this is okay. We also don't model the possibility of someone hex-editing the binary ;) -Hal> On Wed, Feb 24, 2016 at 6:57 PM, Sanjoy Das via llvm-dev < > llvm-dev at lists.llvm.org > wrote:> > Hi all, >> > This is something that came up in the "RFC: Add guard intrinsics to > > > LLVM" thread; and while I'm not exactly blocked on this, figuring > > out > > > a path forward here will be helpful in deciding if we can use the > > > available_externally linkage type to expression certain semantic > > > properties guard intrinsics will have. >> > Let's start with an example that shows that we have a problem > > (direct > > > copy/paste from the guard intrinsics thread). Say we have: >> > ``` > > > void foo() available_externally { > > > %t0 = load atomic %ptr > > > %t1 = load atomic %ptr > > > if (%t0 != %t1) print("X"); > > > } > > > void main() { > > > foo(); > > > print("Y"); > > > } > > > ``` >> > The possible behaviors of the above program are {print("X"), > > > print("Y")} or {print("Y")}. But if we run opt then we have >> > ``` > > > void foo() available_externally readnone nounwind { > > > ;; After CSE'ing the two loads and folding the condition > > > } > > > void main() { > > > foo(); > > > print("Y"); > > > } > > > ``` >> > and some generic reordering >> > ``` > > > void foo() available_externally readnone nounwind { > > > ;; After CSE'ing the two loads and folding the condition > > > } > > > void main() { > > > print("Y"); > > > foo(); // legal since we're moving a readnone nounwind function > > that > > > // was guaranteed to execute (hence can't have UB) > > > } > > > ``` >> > If we do not inline @foo(), and instead re-link the call site in > > @main > > > to some non-optimized copy (or differently optimized copy) of @foo, > > > then it is possible for the program to have the behavior > > {print("Y"); > > > print ("X")}, which was disallowed in the earlier program. >> > In other words, opt refined the semantics of @foo() (i.e. reduced > > the > > > set of behaviors it may have) in ways that would make later > > > optimizations invalid if we de-refine the implementation of @foo(). >> > The above example is clearly fabricated, but such cases can come up > > > even if everything is optimized to the same level. E.g. one of the > > > atomic loads in the unrefined implementation of @foo() could have > > been > > > hidden behind a function call, whose body existed in only one > > module. > > > That module would then be able to refine @foo() to `ret void` but > > > other modules won't. >> > The only solution I can think of is to redefine > > available_externally > > > to mean "the only kind of IPO/IPA you can do over a call to this > > > function is to inline it". Redefining available_externally this way > > > will also let us soundly use it to represent calls to functions > > that > > > have guard intrinsics, since a failed guard intrinsic basically > > > replaces the function with a "very de-refined" implementation (the > > > interpreter). >> > What do you think? I don't think implementing the above above will > > be > > > very difficult, but needless to say, it will still be a fairly > > > non-trivial semantic change (hence I'm not directly jumping to > > > implementation). >> > -- 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-- Hal Finkel Assistant Computational Scientist Leadership Computing Facility Argonne National Laboratory -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20160226/796e1c12/attachment.html>
Gerolf Hoflehner via llvm-dev
2016-Feb-27 06:16 UTC
[llvm-dev] Possible soundness issue with available_externally (split from "RFC: Add guard intrinsics")
> On Feb 25, 2016, at 11:41 AM, James Y Knight via llvm-dev <llvm-dev at lists.llvm.org> wrote: > > While we're talking about this, I'd just mention again that the same issue arises for *normal* functions too, when linked into a shared library: > int foo() { return 1; } > int bar() { return foo(); } > > Now, compare: > clang -fPIC -O1 -S -o - test.c > gcc -fPIC -O1 -S -o - test.c > > GCC will refuse to inline foo into bar, or use any information about foo in compiling bar, because foo is exported in the dynamic symbol table, and thus replaceable via symbol interposition. > > Clang assumes that you won't do that, or that you don't care what happens if you do. It will happily inline. And, in absense of inlining (e.g. if foo is too long to inline), clang will deduce function attributes about foo and rely on those in bar -- despite that the call goes through the PLT and could in fact be an entirely different unrelated implementation (or, for that matter, a differently-optimized version of the same implementation). > > Is that *really* okay?+1 I agree. The problem goes deeper than just dealing with function attributes. The question is what optimizations are allowed for an OS specific preemption model? The function attributes add additional need for clarification. It think at the heart of this difference are assumptions about the OS preemption model. Linux by default assumes that global data/functions are preemptable, so in your example based on that model foo could not be inlined. You should also see gp save and restores around global calls for similar reasons, extra levels of indirections when loading global data etc. An alternative model is to invert the default by requiring preemptable data/functions to be marked. This is the path eg. Windows has chosen with dllimport directives. FWIW, my reading of available_external is that although the function is/can be preempted it still can be inlined since the code of the external function will match the definition in the modulo. The question about legality of other optimizations are similar to the question which optimization is allowed in which preemption model even w/o the attribute. However, I don’t have much experience with the function attributes. -Gerolf> > > On Wed, Feb 24, 2016 at 6:57 PM, Sanjoy Das via llvm-dev <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote: > Hi all, > > This is something that came up in the "RFC: Add guard intrinsics to > LLVM" thread; and while I'm not exactly blocked on this, figuring out > a path forward here will be helpful in deciding if we can use the > available_externally linkage type to expression certain semantic > properties guard intrinsics will have. > > Let's start with an example that shows that we have a problem (direct > copy/paste from the guard intrinsics thread). Say we have: > > ``` > void foo() available_externally { > %t0 = load atomic %ptr > %t1 = load atomic %ptr > if (%t0 != %t1) print("X"); > } > void main() { > foo(); > print("Y"); > } > ``` > > The possible behaviors of the above program are {print("X"), > print("Y")} or {print("Y")}. But if we run opt then we have > > ``` > void foo() available_externally readnone nounwind { > ;; After CSE'ing the two loads and folding the condition > } > void main() { > foo(); > print("Y"); > } > ``` > > and some generic reordering > > ``` > void foo() available_externally readnone nounwind { > ;; After CSE'ing the two loads and folding the condition > } > void main() { > print("Y"); > foo(); // legal since we're moving a readnone nounwind function that > // was guaranteed to execute (hence can't have UB) > } > ``` > > If we do not inline @foo(), and instead re-link the call site in @main > to some non-optimized copy (or differently optimized copy) of @foo, > then it is possible for the program to have the behavior {print("Y"); > print ("X")}, which was disallowed in the earlier program. > > In other words, opt refined the semantics of @foo() (i.e. reduced the > set of behaviors it may have) in ways that would make later > optimizations invalid if we de-refine the implementation of @foo(). > > The above example is clearly fabricated, but such cases can come up > even if everything is optimized to the same level. E.g. one of the > atomic loads in the unrefined implementation of @foo() could have been > hidden behind a function call, whose body existed in only one module. > That module would then be able to refine @foo() to `ret void` but > other modules won't. > > The only solution I can think of is to redefine available_externally > to mean "the only kind of IPO/IPA you can do over a call to this > function is to inline it". Redefining available_externally this way > will also let us soundly use it to represent calls to functions that > have guard intrinsics, since a failed guard intrinsic basically > replaces the function with a "very de-refined" implementation (the > interpreter). > > What do you think? I don't think implementing the above above will be > very difficult, but needless to say, it will still be a fairly > non-trivial semantic change (hence I'm not directly jumping to > implementation). > > > -- Sanjoy > _______________________________________________ > LLVM Developers mailing list > llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org> > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev <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/20160226/a8c0f5ba/attachment-0001.html>
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