[PATCH] xtensa: add -mpreferred-stack-boundary option

[Max Filippov]

2016-05-24  Max Filippov  <jcmvbkbc@gmail.com>
gcc/
	* config/xtensa/xtensa.opt (mpreferred-stack-boundary=): New
	option.
	* config/xtensa/xtensa.h (STACK_BOUNDARY): Redefine as 64 for
	windowed ABI/32 for call0 ABI.
	(PREFERRED_STACK_BOUNDARY): New definition.
	* config/xtensa/xtensa.c (xtensa_function_arg_1,
	xtensa_function_arg_boundary, STACK_BYTES,
	xtensa_gimplify_va_arg_expr): Use PREFERRED_STACK_BOUNDARY
	instead of STACK_BOUNDARY.
	* doc/invoke.texi: Document -mpreferred-stack-boundary.
---
 gcc/config/xtensa/xtensa.c   | 12 +++++++-----
 gcc/config/xtensa/xtensa.h   | 16 +++++++++++++---
 gcc/config/xtensa/xtensa.opt |  4 ++++
 gcc/doc/invoke.texi          | 16 ++++++++++++++++
 4 files changed, 40 insertions(+), 8 deletions(-)

diff --git a/gcc/config/xtensa/xtensa.c b/gcc/config/xtensa/xtensa.c
index 60e5029..332aa81 100644
--- a/gcc/config/xtensa/xtensa.c
+++ b/gcc/config/xtensa/xtensa.c
@@ -2117,7 +2117,8 @@ xtensa_function_arg_1 (cumulative_args_t cum_v, machine_mode mode,
 
   if (type && (TYPE_ALIGN (type) > BITS_PER_WORD))
     {
-      int align = MIN (TYPE_ALIGN (type), STACK_BOUNDARY) / BITS_PER_WORD;
+      int align = MIN (TYPE_ALIGN (type),
+		       PREFERRED_STACK_BOUNDARY) / BITS_PER_WORD;
       *arg_words = (*arg_words + align - 1) & -align;
     }
 
@@ -2158,8 +2159,8 @@ xtensa_function_arg_boundary (machine_mode mode, const_tree type)
   alignment = type ? TYPE_ALIGN (type) : GET_MODE_ALIGNMENT (mode);
   if (alignment < PARM_BOUNDARY)
     alignment = PARM_BOUNDARY;
-  if (alignment > STACK_BOUNDARY)
-    alignment = STACK_BOUNDARY;
+  if (alignment > PREFERRED_STACK_BOUNDARY)
+    alignment = PREFERRED_STACK_BOUNDARY;
   return alignment;
 }
 
@@ -2624,7 +2625,7 @@ xtensa_call_save_reg(int regno)
 /* Return the bytes needed to compute the frame pointer from the current
    stack pointer.  */
 
-#define STACK_BYTES (STACK_BOUNDARY / BITS_PER_UNIT)
+#define STACK_BYTES (PREFERRED_STACK_BOUNDARY / BITS_PER_UNIT)
 #define XTENSA_STACK_ALIGN(LOC) (((LOC) + STACK_BYTES-1) & ~(STACK_BYTES-1))
 
 long
@@ -3184,7 +3185,8 @@ xtensa_gimplify_va_arg_expr (tree valist, tree type, gimple_seq *pre_p,
 
   if (TYPE_ALIGN (type) > BITS_PER_WORD)
     {
-      int align = MIN (TYPE_ALIGN (type), STACK_BOUNDARY) / BITS_PER_UNIT;
+      int align = MIN (TYPE_ALIGN (type),
+		       PREFERRED_STACK_BOUNDARY) / BITS_PER_UNIT;
 
       t = build2 (PLUS_EXPR, integer_type_node, unshare_expr (orig_ndx),
 		  build_int_cst (integer_type_node, align - 1));
diff --git a/gcc/config/xtensa/xtensa.h b/gcc/config/xtensa/xtensa.h
index 82e9900..abcb799 100644
--- a/gcc/config/xtensa/xtensa.h
+++ b/gcc/config/xtensa/xtensa.h
@@ -504,9 +504,19 @@ enum reg_class
    location above the first argument's address.  */
 #define FIRST_PARM_OFFSET(FNDECL) 0
 
-/* Align stack frames on 128 bits for Xtensa.  This is necessary for
-   128-bit datatypes defined in TIE (e.g., for Vectra).  */
-#define STACK_BOUNDARY 128
+/* Align stack frames on 32 bits for call0 ABI or on 64 bits for windowed
+   ABI on Xtensa.  These are the absolute minimums dictated by the hardware
+   (windowed is 64 bits because the entry instruction moves SP in multiples
+   of 8 bytes).  ABIs require alignment to be 128 bits to support datatypes
+   defined in TIE (e.g., for Vectra).  */
+#define STACK_BOUNDARY (TARGET_WINDOWED_ABI ? 64 : 32)
+
+/* Align stack frames on 128 bits in accordance with ABI.  This alignment
+   may be reduced when stack space conservation is preferred over ABI
+   compliance with -mpreferred-stack-boundary option.  */
+#define PREFERRED_STACK_BOUNDARY					\
+	MAX (STACK_BOUNDARY,						\
+	     (BITS_PER_UNIT << xtensa_preferred_stack_boundary))
 
 /* Use a fixed register window size of 8.  */
 #define WINDOW_SIZE (TARGET_WINDOWED_ABI ? 8 : 0)
diff --git a/gcc/config/xtensa/xtensa.opt b/gcc/config/xtensa/xtensa.opt
index ea5c7d5..85e9622 100644
--- a/gcc/config/xtensa/xtensa.opt
+++ b/gcc/config/xtensa/xtensa.opt
@@ -45,3 +45,7 @@ Relax literals in assembler and place them automatically in the text section.
 mserialize-volatile
 Target Report Mask(SERIALIZE_VOLATILE)
 -mno-serialize-volatile	Do not serialize volatile memory references with MEMW instructions.
+
+mpreferred-stack-boundary=
+Target RejectNegative Joined UInteger Var(xtensa_preferred_stack_boundary) Init(4)
+Attempt to keep stack aligned to this power of 2.
diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
index 0241cb5..2e885a8 100644
--- a/gcc/doc/invoke.texi
+++ b/gcc/doc/invoke.texi
@@ -1202,6 +1202,7 @@ See RS/6000 and PowerPC Options.
 -mtext-section-literals  -mno-text-section-literals @gol
 -mauto-litpools  -mno-auto-litpools @gol
 -mtarget-align  -mno-target-align @gol
+-mpreferred-stack-boundary=@var{num} @gol
 -mlongcalls  -mno-longcalls}
 
 @emph{zSeries Options}
@@ -25337,6 +25338,21 @@ treatment of auto-aligned instructions like @code{LOOP}, which the
 assembler always aligns, either by widening density instructions or
 by inserting NOP instructions.
 
+@item -mpreferred-stack-boundary=@var{num}
+@opindex mpreferred-stack-boundary
+Attempt to keep the stack boundary aligned to a 2 raised to @var{num}
+byte boundary.  If @option{-mpreferred-stack-boundary} is not specified,
+the default is 4 (16 bytes or 128 bits).
+
+@strong{Warning:} When generating code for the Xtensa architecture that
+does not use TIE types, @option{-mpreferred-stack-boundary=3} can be
+used to keep the stack boundary aligned to 8 byte boundary in windowed
+ABI, and @option{-mpreferred-stack-boundary=2} can be used to keep the
+stack boundary aligned to 4 byte boundary in call0 ABI.  Since both
+windowed and call0 ABI require 16 byte stack alignment, this is ABI
+incompatible and intended to be used in controlled environment where
+stack space is important limitation.
+
 @item -mlongcalls
 @itemx -mno-longcalls
 @opindex mlongcalls
-- 
2.1.4