[llvm-dev] target ABI: call parameters handling

Jonas Paulsson via llvm-dev llvm-dev at lists.llvm.org
Mon Oct 4 08:05:30 PDT 2021

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?


> 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
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