[cfe-dev] The x64 ABI lowering, direct pass of upper 8-bytes, can it happen?

[Craig Topper via cfe-dev]

Adding an assert(0) inside that if fires on this test case from
clang/test/CodeGenCXX/x86_64-arguments.cpp

// PR5831
// CHECK-LABEL: define void @_Z2f34s3_1(i64 %x.coerce)
struct s3_0 {};
struct s3_1 { struct s3_0 a; long b; };
void f3(struct s3_1 x) {}

~Craig


On Thu, Nov 12, 2020 at 1:06 PM Christoffer Lernö via cfe-dev <
cfe-dev at lists.llvm.org> wrote:

> I apologize for spamming with so many questions, but this really confusing
> me.
>
> In X86_64ABIInfo::classifyArgumentType we have this code:
>
> >   case Integer:
> >     ++neededInt;
> >     // Pick an 8-byte type based on the preferred type.
> >     HighPart = GetINTEGERTypeAtOffset(CGT.ConvertType(Ty), 8, Ty, 8);
> >
> >     if (Lo == NoClass)  // Pass HighPart at offset 8 in memory.
> >       return ABIArgInfo::getDirect(HighPart, 8);
> >     break;
>
> Zooming in on the ”Lo == NoClass”. As far as I can tell this can only
> happen in the case where a struct has padding in front, and the first real
> integer parameter is actually 64 bits in.
>
> I admit I have no idea how to construct such a type in C to trigger this.
> Since Clang has a non-trivial amount of code to handle this I am wondering
> what I’m missing. (I’ve looked through the x86 test cases as well but was
> unable to find a test case for this particular situation. I might have
> overlooked something though)
>
>
> /Christoffer
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