Jonas Paulsson via llvm-dev
2021-Oct-04 15:05 UTC
[llvm-dev] target ABI: call parameters handling
Hi Reid,> I don't think it would be reasonable for the SystemZ backend to abort > every time it is asked to code generate an i32 argument that lacks an > attribute. There are simply too many IR producers (frontends and > instrumentation tools) that will pass unannotated i32 values to calls. > It's not feasible to fix them all, and aborting just means these > producers will have to be updated whenever they are ported to SystemZ > or any other target adopting the same practice. > > I think it would instead be reasonable to pick a safer default for the > target, like always zero extending unannotated parameters on both the > caller side and the callee side. This is less efficient than only > extending once in the caller, but it makes it much easier to retarget > an IR producer over to SystemZ. It makes the attributes into > performance improvements, not correctness requirements. I suspect this > will not actually break passing a 4 byte struct, it just makes it less > efficient. >As Uli explained, this is not an optimization but actually really necessary in order to perform the right extension. I don't think there is any way around fixing this the slow and arduous way :-/> As for the suggestion to add an explicit structarg attribute, can this > be generalized to an attribute that marks the high bits as undef? So, > IR producers will have a portable way to explicitly request the > current behavior. This is kind of like `anyext`, but not really, more > like `undefext`, `uext`, `hiundef` or `calleemustextend`. > > What do you think?I agree that some more general attribute could work as well to mean for a parameter that it is not an integer value and no extension is therefore required per the ABI. The important thing about the name is that whoever builds the call instruction needs to be aware that it is only for non-integer values - the other trap to fall into I guess might be that 'anyext' could be added carelessly on integers when it seems that it might not matter, even when it really does per the ABI... :-)B Maybe the name 'anyext' would make people think that it will be extended one way or the other, which is not true. I think I like your 'hiundef' better... Or maybe 'nonint', or 'noext'? Do you agree we need to do this, and what would the next step be towards getting this in place? If we decide on the attribute to use, I could implement the assertion and start filing bugs and hopefully get some help with them as well... Even though this may not be "feasible", I hope this would eventually lead to having this working correctly, or is there any reason not? /Jonas> On Wed, Sep 29, 2021 at 4:42 AM Jonas Paulsson > <paulsson at linux.vnet.ibm.com> wrote: > > Hi, > > We have many times now had severe problems relating to parameter > passing > where the signext/zeroext attribute has been forgotten when > building the > LLVM I/R call instruction somewhere during compilation. The last > example > of this is MLIR: some function related to async runtime takes an > int32_t > as argument, but the place that builds the call is not adding the > signext attribute on that parameter. As a result wrong code(!) > resulted > on SystemZ and just out of luck we happened to discover this when > building MLIR with gcc (bug hides with clang). Details at > https://bugs.llvm.org/show_bug.cgi?id=51898. > > The first question one might ask about this is why is there no assert > catching this in the SystemZ (or any other target with the same ABI > handling of arguments)? The reason is that there is a third type of > integer parameter: struct passed in register. So if there is > neither a > signext or a zeroext attribute on an i32 argument, it must be assumed > that it contains a struct (of four bytes, for example). > > I personally feel there is something missing here and a shame that > this > very serious problem lives on. I totally understand that people > working > on a target that does not have this ABI requirement may not be aware > that their new code causes wrong-code on some platforms. Therefore I > think it all the more important that they see some test failing as > soon > as they make the mistake. > > In order to implement the assert in the backend one idea might be to > give the struct-in-reg parameter an explicit attribute. Would it be > reasonable/doable to invent a new attribute such as StructArg and > add it > on all such arguments throughout the whole LLVM code base? Then we > could > assert for signext/zeroext/structarg and eliminate this type of > errors. > > Some examples: > > The SystemZ ABI requires that all integer parameters always are to be > extended to full register width (64-bit), either signed or unsigned > depending on the type. > > -- For exampe, this function is loading 32-bits which are passed to > another function: > > int bar(int *a) { > B BB return foo(*a); > } > > => LLVM I/R: > > B BB %0 = load i32, i32* %a > B BB %call = tail call signext i32 @foo(i32 signext %0) > > => SystemZ MachineInstructions: > > B BB %1:gr64bit = LGF %0:addr64bit, 0, $noreg :: (load (s32) from > %ir.a, > !tbaa !2) > B BB $r2d = COPY %1:gr64bit > B BB CallJG @foo, implicit $r2d > > The important mechanism here is that on LLVM I/R the 'signext' > attribute > is added to the parameter %0. That way the backend knows that a sign > extension is needed (there is no other way to know the > signedness), and > emits the sign-extending load (LGF). > > -- A matching example, on the callee side would be: > > long bar(int a) { > B BB return ((long) a); > } > > => > > B BB %conv = sext i32 %a to i64 > B BB ret i64 %conv > > => > > B BB Return implicit $r2d > > The 32-bit ingoing parameter is per the ABI required to already have > been sign-extended so the backend can simply do nothing. NB! This > leads > to wrong code if the caller forgets about that :-) > > -- A struct in register: > > struct S { > B BB short el0; > B BB short el1; > }; > > int foo(struct S arg); > > void bar(short c) { > B BB struct S a = {c, c + 1}; > B BB foo (a); > } > > => LLVM I/R > > define void @bar(i16 signext %c) local_unnamed_addr #0 { > > B BB %add = add i16 %c, 1 > B BB %a.sroa.4.0.insert.ext = zext i16 %add to i32 > B BB %a.sroa.0.0.insert.ext = zext i16 %c to i32 > B BB %a.sroa.0.0.insert.shift = shl nuw i32 %a.sroa.0.0.insert.ext, 16 > B BB %a.sroa.0.0.insert.insert = or i32 %a.sroa.0.0.insert.shift, > %a.sroa.4.0.insert.ext > B BB %call = tail call signext i32 @foo(i32 > %a.sroa.0.0.insert.insert) #2 > B BB ret void > } > > The i32 passed to foo does not have any attribute set, and the > backend > does not extend it. An 'int' gets the same treatment without the > signext > attribute set, which is then wrong... :-( > > /Jonas > >-------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20211004/9c0fedeb/attachment.html>
Reid Kleckner via llvm-dev
2021-Oct-06 02:31 UTC
[llvm-dev] target ABI: call parameters handling
On Mon, Oct 4, 2021 at 8:05 AM Jonas Paulsson <paulsson at linux.vnet.ibm.com> wrote:> Hi Reid, > > I don't think it would be reasonable for the SystemZ backend to abort > every time it is asked to code generate an i32 argument that lacks an > attribute. There are simply too many IR producers (frontends and > instrumentation tools) that will pass unannotated i32 values to calls. It's > not feasible to fix them all, and aborting just means these producers will > have to be updated whenever they are ported to SystemZ or any other target > adopting the same practice. > > I think it would instead be reasonable to pick a safer default for the > target, like always zero extending unannotated parameters on both the > caller side and the callee side. This is less efficient than only extending > once in the caller, but it makes it much easier to retarget an IR producer > over to SystemZ. It makes the attributes into performance improvements, not > correctness requirements. I suspect this will not actually break passing a > 4 byte struct, it just makes it less efficient. > > As Uli explained, this is not an optimization but actually really > necessary in order to perform the right extension. I don't think there is > any way around fixing this the slow and arduous way :-/ >Well, the annotation isn't strictly necessary. The backend can choose to emit an extension for short integer arguments in the absence of annotations. The value would be to reduce the number of SystemZ-specific changes to IR producers. It would be a shame if i32 arguments just didn't work out of the box on SystemZ. That will be an ongoing source of IR producer bugs. In any case, I don't feel like my argument is all that compelling. If you and the SystemZ owners feel like this is the best plan, go for it. The same problem exists for i16 and i8 arguments on x86, BTW: https://gcc.godbolt.org/z/7dds6sfv8 These are much less common argument types, but you might consider expanding the scope of this proposal.> As for the suggestion to add an explicit structarg attribute, can this be > generalized to an attribute that marks the high bits as undef? So, IR > producers will have a portable way to explicitly request the current > behavior. This is kind of like `anyext`, but not really, more like > `undefext`, `uext`, `hiundef` or `calleemustextend`. > > What do you think? > > I agree that some more general attribute could work as well to mean for a > parameter that it is not an integer value and no extension is therefore > required per the ABI. The important thing about the name is that whoever > builds the call instruction needs to be aware that it is only for > non-integer values - the other trap to fall into I guess might be that > 'anyext' could be added carelessly on integers when it seems that it might > not matter, even when it really does per the ABI... :-) Maybe the name > 'anyext' would make people think that it will be extended one way or the > other, which is not true. I think I like your 'hiundef' better... Or maybe > 'nonint', or 'noext'? > > Do you agree we need to do this, and what would the next step be towards > getting this in place? If we decide on the attribute to use, I could > implement the assertion and start filing bugs and hopefully get some help > with them as well... Even though this may not be "feasible", I hope this > would eventually lead to having this working correctly, or is there any > reason not? >I think I like `noext`, it fits well with the existing attributes. As for next steps, we need to settle on the name. If you like `noext`, send a patch, CC some other folks and see if they like the name. -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20211005/d14a26ff/attachment.html>