Rafael Espíndola via llvm-dev
2016-Oct-25 21:55 UTC
[llvm-dev] RFC: Absolute or "fixed address" symbols as immediate operands
> You get the code you want with > > @foo = external hidden global i8 > > or > > @foo = external protected global i8Sorry, not quiet. What you get is leaq foo(%rip), %rax Since it is still assuming foo is a position in the file. So yes, we need a way to declare an absolute value. If we can declare it, we may as well use the same construct to define it. I guess the only issue then is that the current implementation of alias becomes even more needlessly generic :-( Cheers, Rafael
Peter Collingbourne via llvm-dev
2016-Oct-26 01:52 UTC
[llvm-dev] RFC: Absolute or "fixed address" symbols as immediate operands
On Tue, Oct 25, 2016 at 2:55 PM, Rafael Espíndola < rafael.espindola at gmail.com> wrote:> > You get the code you want with > > > > @foo = external hidden global i8 > > > > or > > > > @foo = external protected global i8 > > Sorry, not quiet. What you get is > > leaq foo(%rip), %rax > > Since it is still assuming foo is a position in the file. > > So yes, we need a way to declare an absolute value. If we can declare > it, we may as well use the same construct to define it. > > I guess the only issue then is that the current implementation of > alias becomes even more needlessly generic :-( > > Cheers, > Rafael >So, to summarise the overall consensus on this thread: - We should introduce a GlobalConstant class which supports definitions and declarations - It should derive from GlobalValue - No type changes for GlobalValue (i.e. still required to be pointer type) - To communicate the range we should use !range metadata and support it on GlobalConstant as well as GlobalVariable Although I am not convinced that this is the right representation I appear to be in the minority here, so if this seems reasonable to folks I will go ahead and implement it. Thanks, -- -- Peter -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20161025/5abc33d4/attachment.html>
Chandler Carruth via llvm-dev
2016-Oct-26 05:16 UTC
[llvm-dev] RFC: Absolute or "fixed address" symbols as immediate operands
On Tue, Oct 25, 2016 at 6:52 PM Peter Collingbourne <peter at pcc.me.uk> wrote:> On Tue, Oct 25, 2016 at 2:55 PM, Rafael Espíndola < > rafael.espindola at gmail.com> wrote: > > > You get the code you want with > > > > @foo = external hidden global i8 > > > > or > > > > @foo = external protected global i8 > > Sorry, not quiet. What you get is > > leaq foo(%rip), %rax > > Since it is still assuming foo is a position in the file. > > So yes, we need a way to declare an absolute value. If we can declare > it, we may as well use the same construct to define it. > > I guess the only issue then is that the current implementation of > alias becomes even more needlessly generic :-( > > Cheers, > Rafael > > > So, to summarise the overall consensus on this thread: > - We should introduce a GlobalConstant class which supports definitions > and declarations > - It should derive from GlobalValue > - No type changes for GlobalValue (i.e. still required to be pointer type) > - To communicate the range we should use !range metadata and support it on > GlobalConstant as well as GlobalVariable > > Although I am not convinced that this is the right representation I appear > to be in the minority here, so if this seems reasonable to folks I will go > ahead and implement it. >FWIW, I really dislike this plan. We are essentially saying we have a GlobalConstant class which is a pointer, but doesn't point to anything. Instead, it is a pointer that will have a fixed "constant" address that can be manipulated and/or exploited to have a very particular integer value via ptrtoint. I feel like we are going to tremendous lengths to avoid having a non-pointer global value despite ... having a non-pointer global value. I would like to understand why it is so problematic to just model this directly and explicitly in the IR...> > Thanks, > -- > -- > Peter >-------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20161026/3aa9dbb6/attachment.html>
Chris Lattner via llvm-dev
2016-Oct-26 05:48 UTC
[llvm-dev] RFC: Absolute or "fixed address" symbols as immediate operands
Responding to both of your emails in one, sorry for the delay:> On Oct 25, 2016, at 11:20 AM, Peter Collingbourne <peter at pcc.me.uk> wrote: > I think there are a couple of additional considerations we should make here: > What are we trying to model? To me it's clear that GlobalConstant is for modelling integers, not pointers. That alone may not necessarily be enough to motivate a representational change, but…I understand where you’re coming from, but I think we’re modeling three different things, and disagreeing about how to clump them together. The three things I see in flight are: 1) typical globals that are laid out in some unknown way in the address space. 2) globals that may be tied to a specific knowable address range due to a limited compilation model (e.g. a deeply embedded core) that fits into an immedaite range (e.g. 0…255, 0…65536, etc). 3) Immediates that are treated as symbolic for CFI’s perspective (so they can’t just be used as a literal immediate) that are resolved at link time, but are known to have limited range. There is also "4) immediates with an obvious known value”, but those are obviously ConstantInt’s and not interesting to discuss here. The design I’m arguing for is to clump #2 and #3 into the same group. This can be done one of two different ways, but both ways use the same “declaration side” reference, which has a !range metadata attached to it. The three approaches I see are: a) Introduce a new GlobalConstant definition, whose value is the concrete address that the linker should resolve. b) Use an alias as the definition, whose body is a ptrtoint constant of the same value. c) Use a zero size globalvariable with a range metadata specifying the exact address decided. I’m not very knowledgable about why approach b won’t work, but if it could, it seems preferable because it fits in with our current model. After that, approach C seems like a nice fallback because it again doesn’t require major extensions to our model. Finally, approach A seems like the most explicit model, but has the disadvantage of introducing a new IR concept that will be rarely used (and thus prone to bugs due to it not being widely tested).> If we make our representation fit the model, does that improve the quality of the code? There's clearly been a long standing assumption that GlobalValues are of pointer type. Breaking that assumption has in fact found potential bugs (see e.g. my changes to ValueTracking.cpp in D25930, as well as D25917 which was a requirement for the representational change), and seems like it may help prevent bugs in the future, so I'd say that the answer is most likely yes.I’d argue the other side of it. The quality of the code is higher if we have invariants (like all globals are pointers) because that simplifies assumptions by eliminating cases where “is a pointer” appears to be true, but isn’t actually true in all cases. I’m not an expert on CFI or how widely it will ultimately impact the compiler hacker consciousness, but I’m pretty sure that the current model for globals and functions will remain more prominent. If you choose to break this invariant, you’ll be continually swimming upstream against assumptions made throughout the compiler, both in code written today but also in code written in the future. On Oct 25, 2016, at 6:52 PM, Peter Collingbourne <peter at pcc.me.uk> wrote:> > So, to summarise the overall consensus on this thread: > - We should introduce a GlobalConstant class which supports definitions and declarationsWhy would globalconstant need to support declarations? Seems like an external global should be able to handle this case.> - It should derive from GlobalValue > - No type changes for GlobalValue (i.e. still required to be pointer type) > - To communicate the range we should use !range metadata and support it on GlobalConstant as well as GlobalVariable > > Although I am not convinced that this is the right representation I appear to be in the minority here, so if this seems reasonable to folks I will go ahead and implement it.I’m not trying to steamroll you over: what is your specific concern? -Chris -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20161025/1d590327/attachment.html>
Reid Kleckner via llvm-dev
2016-Oct-26 16:14 UTC
[llvm-dev] RFC: Absolute or "fixed address" symbols as immediate operands
On Tue, Oct 25, 2016 at 6:52 PM, Peter Collingbourne <peter at pcc.me.uk> wrote:> So, to summarise the overall consensus on this thread: > - We should introduce a GlobalConstant class which supports definitions > and declarations > - It should derive from GlobalValue > - No type changes for GlobalValue (i.e. still required to be pointer type) > - To communicate the range we should use !range metadata and support it on > GlobalConstant as well as GlobalVariable > > Although I am not convinced that this is the right representation I appear > to be in the minority here, so if this seems reasonable to folks I will go > ahead and implement it. >I could go either way on GlobalConstant being a pointer or not. Keeping the "globals are points" invariant is nice, but making LLVM IR more accurately model object file formats is also good. If I had to choose, I think I would make GlobalConstant a pointer. I don't think there are many interesting mid-level optimizations that this prevents. One change I would make is to push the !range metadata into the GlobalConstant object. The range information is what really unlocks the interesting ISel optimizations. -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20161026/6e2f352c/attachment.html>
Apparently Analagous Threads
- RFC: Absolute or "fixed address" symbols as immediate operands
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- RFC: Absolute or "fixed address" symbols as immediate operands