CODEWITHSUNDEEP
cclangllvm
brooks94
brooks94

Reputation: 3936

Clang splits function arguments

If you compile the following with "clang -S -emit-llvm struct.c"

struct _mystruct {
  long long int a;
  long long int b;
};

struct _mystruct foo(struct _mystruct s) {
  s.a += 1;
  return s;
}

int main(void) {
  struct _mystruct s;
  s.a = 8;
  s.b = 9;
  s = foo(s);
  return s.a;
}

... you get (among other things):

define { i64, i64 } @foo(i64 %s.coerce0, i64 %s.coerce1) #0 {

Why does clang split the argument to foo in two? Is there any way I can prevent it from doing that? I want to be able to call it from other LLVM generated code that expects only one argument to foo.

Upvotes: 1

Views: 389

Answers (1)

Eli Bendersky
Eli Bendersky

Reputation: 273844

Since LLVM has no way to represent it, Clang encodes the platform ABI this way. In this particular example, it's struct passing by-value which is extremely ABI specific. You will notice this if you provide different target triple to clang - you'll notice that the emitted code is different. I assume from the question this is run on a x64 machine where structs can be passed in registers.

Upvotes: 1

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