tree d7a79c2136df260ecca0d746a65f33bfb9961112
parent d1c84d3c0206abdf8787dc5c00aaf3b016ed3226
author Daniel Dunbar <daniel@zuster.org> 1331341438 +0000
committer Daniel Dunbar <daniel@zuster.org> 1331341438 +0000

IRgen/ABI/x86_64: Avoid passing small structs using byval sometimes.

 - We do this when it is easy to determine that the backend will pass them on
   the stack properly by itself.

Currently LLVM codegen is really bad in some cases with byval, for example, on
the test case here (which is derived from Sema code, which likes to pass
SourceLocations around)::

  struct s47 { unsigned a; };
  void f47(int,int,int,int,int,int,struct s47);
  void test47(int a, struct s47 b) { f47(a, a, a, a, a, a, b); }

we used to emit code like this::

  ...
  movl	%esi, -8(%rbp)
  movl	-8(%rbp), %ecx
  movl	%ecx, (%rsp)
  ...

to handle moving the struct onto the stack, which is just appalling.

Now we generate::

  movl	%esi, (%rsp)

which seems better, no?

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@152462 91177308-0d34-0410-b5e6-96231b3b80d8