implement -msave-args in clang/llvm, like the sun did for gcc

this is a bit different to gcc as gcc likes to use movs to move
stuff on and off the stack, and directly updates the stack pointers
with add and sub instructions. llvm prefers to use push and pop
instructions, is a lot more careful about keeping track of how
much stuff is currently on the stack, and generally pops the frame
pointer rather than do maths on it.

-msave-args adds a bunch of pushes as the first thing a function
prologue does. to keep the stack aligned, if there's an odd number
of arguments to the function it pushes the first one again to put
the frame back on a 16 byte boundary.

to undo the pushes the frame pointer needs to be updated in function
epilogues. clang emits a series of pops to fix up the registers on
the way out, but popping saved arguments is a waste of time and
harmful to actual data in the function. rather than add an offset
to the stack pointer, -msave-args emits a leaveq operation to fix
up the frame again. leaveq is effectively mov rbp,rsp; pop rbp, and
is a single byte, meaning there's less potential for gadgets compared
to a direct add to rsp, or an explicit mov rbp,rsp.

the only thing missing compared to the gcc implementation is adding
the SUN_amd64_parmdump dwarf flag to affected functions. if someone
can tell me how to add that from the frame lowering code, let me
know.

when enabled in kernel builds again, this will provide useful
arguments in ddb stack traces again.

1 files changed, 6 insertions, 0 deletions

diff --git a/gnu/llvm/lib/Target/X86/X86MachineFunctionInfo.h b/gnu/llvm/lib/Target/X86/X86MachineFunctionInfo.h
index e1183bd1479..393abe698db 100644
--- a/gnu/llvm/lib/Target/X86/X86MachineFunctionInfo.h
+++ b/gnu/llvm/lib/Target/X86/X86MachineFunctionInfo.h
@@ -41,6 +41,9 @@ class X86MachineFunctionInfo : public MachineFunctionInfo {
   /// stack frame in bytes.
   unsigned CalleeSavedFrameSize = 0;
 
+  // SaveArgSize - Number of register arguments saved on the stack
+  unsigned SaveArgSize = 0;
+
   /// BytesToPopOnReturn - Number of bytes function pops on return (in addition
   /// to the space used by the return address).
   /// Used on windows platform for stdcall & fastcall name decoration
@@ -124,6 +127,9 @@ public:
   unsigned getCalleeSavedFrameSize() const { return CalleeSavedFrameSize; }
   void setCalleeSavedFrameSize(unsigned bytes) { CalleeSavedFrameSize = bytes; }
 
+  unsigned getSaveArgSize() const { return SaveArgSize; }
+  void setSaveArgSize(unsigned bytes) { SaveArgSize = bytes; }
+
   unsigned getBytesToPopOnReturn() const { return BytesToPopOnReturn; }
   void setBytesToPopOnReturn (unsigned bytes) { BytesToPopOnReturn = bytes;}