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?

llvm-svn: 152462

2 files changed, 15 insertions, 4 deletions

diff --git a/clang/test/CodeGen/complex-indirect.c b/clang/test/CodeGen/complex-indirect.c
index 45eb1954842..0daa970e760 100644
--- a/clang/test/CodeGen/complex-indirect.c
+++ b/clang/test/CodeGen/complex-indirect.c
@@ -1,10 +1,12 @@
-// RUN: %clang_cc1 -emit-llvm %s -o - -triple=x86_64-apple-darwin10 | FileCheck %s
+// RUN: %clang_cc1 -emit-llvm %s -o %t -triple=x86_64-apple-darwin10
+// RUN: FileCheck < %t %s
 
-// Make sure this doesn't crash, and that we don't generate a byval alloca
-// with insufficient alignment.
+// Make sure this doesn't crash. We used to generate a byval here and wanted to
+// verify a valid alignment, but we now realize we can use an i16 and let the
+// backend guarantee the alignment.
 
 void a(int,int,int,int,int,int,__complex__ char);
 void b(__complex__ char *y) { a(0,0,0,0,0,0,*y); }
 // CHECK: define void @b
 // CHECK: alloca { i8, i8 }*, align 8
-// CHECK: call void @a(i32 0, i32 0, i32 0, i32 0, i32 0, i32 0, { i8, i8 }* byval align 8
+// CHECK: call void @a(i32 0, i32 0, i32 0, i32 0, i32 0, i32 0, i16 {{.*}})
diff --git a/clang/test/CodeGen/x86_64-arguments.c b/clang/test/CodeGen/x86_64-arguments.c
index 8e7119ef2c9..f73e1f026a8 100644
--- a/clang/test/CodeGen/x86_64-arguments.c
+++ b/clang/test/CodeGen/x86_64-arguments.c
@@ -345,3 +345,12 @@ void test45() { f45(x45); }
 typedef float v46 __attribute((vector_size(8)));
 void f46(v46,v46,v46,v46,v46,v46,v46,v46,v46,v46);
 void test46() { v46 x = {1,2}; f46(x,x,x,x,x,x,x,x,x,x); }
+
+// Check that we pass the struct below without using byval, which helps out
+// codegen.
+//
+// CHECK: @test47
+// CHECK: call void @f47(i32 {{.*}}, i32 {{.*}}, i32 {{.*}}, i32 {{.*}}, i32 {{.*}}, i32 {{.*}}, i32 {{.*}})
+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); }