From cd79652e8e32b069e40e56716464ac72e002af51 Mon Sep 17 00:00:00 2001
From: Vlad Brezae <brezaevlad@gmail.com>
Date: Tue, 3 Sep 2019 11:21:56 +0300
Subject: [PATCH] [interp] Remove varargs from InterpFrame and recompute it
 instead (mono/mono#16598)

It is a waste of stack space.

Commit migrated from https://github.com/mono/mono/commit/f491af13f482fb42313aa5919ef6e2a030501994
---
 src/mono/mono/mini/interp/interp-internals.h |  1 -
 src/mono/mono/mini/interp/interp.c           | 19 +++++++++++++------
 src/mono/mono/mini/interp/transform.c        |  4 +++-
 3 files changed, 16 insertions(+), 8 deletions(-)

diff --git a/src/mono/mono/mini/interp/interp-internals.h b/src/mono/mono/mini/interp/interp-internals.h
index 3dcb0de..f359f1c 100644
--- a/src/mono/mono/mini/interp/interp-internals.h
+++ b/src/mono/mono/mini/interp/interp-internals.h
@@ -131,7 +131,6 @@ struct _InterpFrame {
 	InterpFrame *parent; /* parent */
 	InterpMethod  *imethod; /* parent */
 	stackval       *retval; /* parent */
-	char           *varargs;
 	stackval       *stack_args; /* parent */
 	stackval       *stack;
 	/*
diff --git a/src/mono/mono/mini/interp/interp.c b/src/mono/mono/mini/interp/interp.c
index 066f34d..be74296 100644
--- a/src/mono/mono/mini/interp/interp.c
+++ b/src/mono/mono/mini/interp/interp.c
@@ -2355,14 +2355,16 @@ do_transform_method (InterpFrame *frame, ThreadContext *context)
 }
 
 static MONO_NEVER_INLINE guchar*
-copy_varargs_vtstack (MonoMethodSignature *csig, stackval *sp, guchar *vt_sp)
+copy_varargs_vtstack (MonoMethodSignature *csig, stackval *sp, guchar *vt_sp_start)
 {
 	stackval *first_arg = sp - csig->param_count;
+	guchar *vt_sp = vt_sp_start;
 
 	/*
 	 * We need to have the varargs linearly on the stack so the ArgIterator
 	 * can iterate over them. We pass the signature first and then copy them
-	 * one by one on the vtstack.
+	 * one by one on the vtstack. At the end we pass the original vt_stack
+	 * so the callee (MINT_ARGLIST) can find the varargs space.
 	 */
 	*(gpointer*)vt_sp = csig;
 	vt_sp += sizeof (gpointer);
@@ -2376,7 +2378,12 @@ copy_varargs_vtstack (MonoMethodSignature *csig, stackval *sp, guchar *vt_sp)
 		vt_sp += arg_size;
 	}
 
-	return (guchar*)ALIGN_PTR_TO (vt_sp, MINT_VT_ALIGNMENT);
+	vt_sp += sizeof (gpointer);
+	vt_sp = (guchar*)ALIGN_PTR_TO (vt_sp, MINT_VT_ALIGNMENT);
+
+	((gpointer*)vt_sp) [-1] = vt_sp_start;
+
+	return vt_sp;
 }
 
 /*
@@ -3243,6 +3250,8 @@ mono_interp_store_remote_field_vt (InterpFrame* frame, const guint16* ip, stackv
  * If BASE_FRAME is not NULL, copy arguments/locals from BASE_FRAME.
  * The ERROR argument is used to avoid declaring an error object for every interp frame, its not used
  * to return error information.
+ *
+ * Currently this method uses 0x88 of stack space on 64bit gcc. Make sure to keep it under control.
  */
 static void
 interp_exec_method_full (InterpFrame *frame, ThreadContext *context, FrameClauseArgs *clause_args, MonoError *error)
@@ -3341,9 +3350,8 @@ main_loop:
 			++sp;
 			MINT_IN_BREAK;
 		MINT_IN_CASE(MINT_ARGLIST)
-			g_assert (frame->varargs);
 			sp->data.p = vt_sp;
-			*(gpointer*)sp->data.p = frame->varargs;
+			*(gpointer*)sp->data.p = ((gpointer*)frame->retval->data.p) [-1];
 			vt_sp += ALIGN_TO (sizeof (gpointer), MINT_VT_ALIGNMENT);
 			++ip;
 			++sp;
@@ -3630,7 +3638,6 @@ main_loop:
 			csig = (MonoMethodSignature*) frame->imethod->data_items [* (guint16*) (ip + 2)];
 			/* Push all vararg arguments from normal sp to vt_sp together with the signature */
 			num_varargs = csig->param_count - csig->sentinelpos;
-			child_frame.varargs = (char*) vt_sp;
 			vt_sp = copy_varargs_vtstack (csig, sp, vt_sp);
 
 			ip += 3;
diff --git a/src/mono/mono/mini/interp/transform.c b/src/mono/mono/mini/interp/transform.c
index 1451915..b735705 100644
--- a/src/mono/mono/mini/interp/transform.c
+++ b/src/mono/mono/mini/interp/transform.c
@@ -2070,8 +2070,10 @@ interp_transform_call (TransformData *td, MonoMethod *method, MonoMethod *target
 		for (i = csignature->sentinelpos; i < csignature->param_count; ++i) {
 			int align, arg_size;
 			arg_size = mono_type_stack_size (csignature->params [i], &align);
-			vararg_stack += arg_size;
+			vararg_stack += ALIGN_TO (arg_size, align);
 		}
+		/* allocate space for the pointer to varargs space start */
+		vararg_stack += sizeof (gpointer);
 		vt_stack_used += ALIGN_TO (vararg_stack, MINT_VT_ALIGNMENT);
 		PUSH_VT (td, vararg_stack);
 	}
-- 
2.7.4