[RFC PATCH v1 4/4] arm: Support for Static Trampolines

[madvenka@linux.microsoft.com]

From: "Madhavan T. Venkataraman" <madvenka@linux.microsoft.com>

	- Define the arch-specific initialization function ffi_tramp_arch ()
	  that returns trampoline size information to common code.

	- Define the trampoline code and data mapping sizes.

	- Introduce a tiny amount of code at the beginning of each ABI
	  handler to retrieve the information saved by the trampoline on
	  stack.

	- Define the trampoline code table statically.

	- Call ffi_closure_tramp_init () to initialize static trampoline
	  parameters from ffi_prep_closure_loc ().

Signed-off-by: Madhavan T. Venkataraman <madvenka@linux.microsoft.com>
---
 src/arm/ffi.c      | 16 ++++++++++++++++
 src/arm/internal.h | 10 ++++++++++
 src/arm/sysv.S     | 37 +++++++++++++++++++++++++++++++++++++
 3 files changed, 63 insertions(+)

diff --git a/src/arm/ffi.c b/src/arm/ffi.c
index 0058390..6529803 100644
--- a/src/arm/ffi.c
+++ b/src/arm/ffi.c
@@ -612,6 +612,9 @@ ffi_prep_closure_loc (ffi_closure * closure,
   config[1] = closure_func;
 #else
 
+  if (ffi_closure_tramp_set_parms (closure, closure_func))
+    goto out;
+
 #ifndef _M_ARM
   memcpy(closure->tramp, ffi_arm_trampoline, 8);
 #else
@@ -633,6 +636,7 @@ ffi_prep_closure_loc (ffi_closure * closure,
 #else
   *(void (**)(void))(closure->tramp + 8) = closure_func;
 #endif
+out:
 #endif
 
   closure->cif = cif;
@@ -873,4 +877,16 @@ layout_vfp_args (ffi_cif * cif)
     }
 }
 
+#if defined(FFI_EXEC_STATIC_TRAMP)
+void *
+ffi_tramp_arch (size_t *tramp_size, size_t *map_size)
+{
+  extern void *trampoline_code_table;
+
+  *tramp_size = ARM_TRAMP_SIZE;
+  *map_size = ARM_TRAMP_MAP_SIZE;
+  return &trampoline_code_table;
+}
+#endif
+
 #endif /* __arm__ or _M_ARM */
diff --git a/src/arm/internal.h b/src/arm/internal.h
index 6cf0b2a..fa8ab0b 100644
--- a/src/arm/internal.h
+++ b/src/arm/internal.h
@@ -5,3 +5,13 @@
 #define ARM_TYPE_INT	4
 #define ARM_TYPE_VOID	5
 #define ARM_TYPE_STRUCT	6
+
+#if defined(FFI_EXEC_STATIC_TRAMP)
+/*
+ * For the trampoline table mapping, a mapping size of 4K (base page size)
+ * is chosen.
+ */
+#define ARM_TRAMP_MAP_SHIFT	12
+#define ARM_TRAMP_MAP_SIZE	(1 << ARM_TRAMP_MAP_SHIFT)
+#define ARM_TRAMP_SIZE		20
+#endif
diff --git a/src/arm/sysv.S b/src/arm/sysv.S
index 74bc53f..f0a435f 100644
--- a/src/arm/sysv.S
+++ b/src/arm/sysv.S
@@ -227,6 +227,10 @@ ARM_FUNC_END(ffi_go_closure_SYSV)
 ARM_FUNC_START(ffi_closure_SYSV)
 	UNWIND(.fnstart)
 	cfi_startproc
+#if defined(FFI_EXEC_STATIC_TRAMP)
+	ldr	ip, [sp, #4]
+	add	sp, sp, 8
+#endif
 	stmdb	sp!, {r0-r3}			@ save argument regs
 	cfi_adjust_cfa_offset(16)
 
@@ -274,6 +278,10 @@ ARM_FUNC_END(ffi_go_closure_VFP)
 ARM_FUNC_START(ffi_closure_VFP)
 	UNWIND(.fnstart)
 	cfi_startproc
+#if defined(FFI_EXEC_STATIC_TRAMP)
+	ldr	ip, [sp, #4]
+	add	sp, sp, 8
+#endif
 	stmdb	sp!, {r0-r3}			@ save argument regs
 	cfi_adjust_cfa_offset(16)
 
@@ -354,6 +362,35 @@ E(ARM_TYPE_STRUCT)
 	cfi_endproc
 ARM_FUNC_END(ffi_closure_ret)
 
+#if defined(FFI_EXEC_STATIC_TRAMP)
+/*
+ * The trampoline uses register ip (r12). It saves the original value of ip
+ * on the stack.
+ *
+ * The trampoline has two parameters - target code to jump to and data for
+ * the target code. The trampoline extracts the parameters from its parameter
+ * block (see tramp_table_map()). The trampoline saves the data address on
+ * the stack. Finally, it jumps to the target code.
+ *
+ * The target code can choose to:
+ *
+ * - restore the value of ip
+ * - load the data address in a register
+ * - restore the stack pointer to what it was when the trampoline was invoked.
+ */
+	.align	ARM_TRAMP_MAP_SHIFT
+ARM_FUNC_START(trampoline_code_table)
+	.rept	ARM_TRAMP_MAP_SIZE / ARM_TRAMP_SIZE
+	sub	sp, sp, #8		/* Make space on the stack */
+	str	ip, [sp]		/* Save ip on stack */
+	ldr	ip, [pc, #4080]		/* Copy data into ip */
+	str	ip, [sp, #4]		/* Save data on stack */
+	ldr	pc, [pc, #4076]		/* Copy code into PC */
+	.endr
+ARM_FUNC_END(trampoline_code_table)
+	.align	ARM_TRAMP_MAP_SHIFT
+#endif /* FFI_EXEC_STATIC_TRAMP */
+
 #if FFI_EXEC_TRAMPOLINE_TABLE
 
 #ifdef __MACH__
-- 
2.25.1