[ffi] isLeaf changes the value returned by calling lookupFunction.

[modulovalue]

For context see: #52692

@dcharkes setting isLeaf to true improves the performance by almost 50%. However, calling the result of lookupFunction appears to return a different value when isLeaf is set to true. Is that expected?

.p2align 2
.global _popcount
_popcount:
    // TODO
    mov x0, x1
    ret

 * Writing file to: /Users/modulovalue/Desktop/hello_assembly/hello_assembly/bin/popcount/output/hello_dylib.s'
 * Running command: 'as -arch arm64 /Users/modulovalue/Desktop/hello_assembly/hello_assembly/bin/popcount/output/hello_dylib.s -o /Users/modulovalue/Desktop/hello_assembly/hello_assembly/bin/popcount/output/hello_dylib.o'
 * Running command: 'clang -current_version 1.0 -compatibility_version 1.0 -dynamiclib -o /Users/modulovalue/Desktop/hello_assembly/hello_assembly/bin/popcount/output/hello_dylib.dylib /Users/modulovalue/Desktop/hello_assembly/hello_assembly/bin/popcount/output/hello_dylib.o'

void run(
  final DynamicLibrary dylib,
) {
  final popcount_leaf =
      dylib.lookupFunction<Uint32 Function(Uint32), int Function(int)>(
    'popcount',
    isLeaf: true,
  );
  final popcount_notleaf =
      dylib.lookupFunction<Uint32 Function(Uint32), int Function(int)>(
    'popcount',
    isLeaf: false,
  );
  print(popcount_notleaf(100000));
  print(popcount_leaf   (100000));
}

outputs:

100000
43032064

on: Dart SDK version: 3.1.0-163.1.beta (beta) (Wed Jun 7 05:06:41 2023 -0700) on "macos_arm64"

[dcharkes]

However, calling the result of lookupFunction appears to return a different value when isLeaf is set to true. Is that expected?

Your function signature takes one integer and returns one integer.

The calling convention in C on MacOS is to pass the argument in x0 and return the value in x0.

sdk/runtime/vm/constants_arm64.cc

Line 33 in 0f6e13a

R0, R1, R2, R3, R4, R5, R6, R7,

sdk/runtime/vm/constants_arm.h

Line 730 in 0f6e13a

static constexpr Register kReturnReg = R0;

Therefore, x1 is undefined.

You can explore calling conventions with

sdk/runtime/vm/constants_arm64.cc

Line 33 in 0f6e13a

R0, R1, R2, R3, R4, R5, R6, R7,

The fact that it works without isLeaf is likely due to somehow the x1 register also being populated with the first argument (maybe the Dart compiler shuffles some values around right before the FFI call).

If you're going to write assembly by hand, it needs to follow the C ABI.

Nit: you're using int32_t while using x registers, w registers are 32 bits wide. So your move is copying the upper 4 bytes as well.

[modulovalue]

So everything that violates the established calling conventions is undefined behavior and not a bug, got it. Thank you for your help.