Add -mgrow-frame-downwards

Grow the local frame down instead of up for mips16 code size.

By growing the frame downwards we get spill slots created at the lowest
address rather than highest address in a local frame. The benefit being
that when the frame is large the spill slots can still be accessed using
a 16bit instruction whereas it is less important for large local
variables to be accessed using short instructions as they are (probably)
accessed less frequently.

This is default on for MIPS16.

gcc/
	* config/mips/mips.h (FRAME_GROWS_DOWNWARD) Allow the frame to
	grow downwards for mips16 when -mgrow-frame-downwards is set.
	* config/mips/mips.opt: Add -mgrow-frame-downwards option.

Author: mfortune

gcc/config/mips/mips.h

@@ -2363,8 +2363,14 @@ enum reg_class
 
 #define STACK_GROWS_DOWNWARD 1
 
-#define FRAME_GROWS_DOWNWARD (flag_stack_protect != 0			\
-			      || (flag_sanitize & SANITIZE_ADDRESS) != 0)
+/* Growing the frame downwards allows us to put spills closest to
+   the stack pointer which is good as they are likely to be accessed
+   frequently.  We can also arrange for normal stack usage to place
+   scalars last so that they too are close to the stack pointer.  */
+#define FRAME_GROWS_DOWNWARD ((TARGET_MIPS16			    \
+			       && TARGET_FRAME_GROWS_DOWNWARDS)     \
+			      || (flag_stack_protect != 0	    \
+				  || (flag_sanitize & SANITIZE_ADDRESS) != 0))
 
 /* Size of the area allocated in the frame to save the GP.  */
 

gcc/config/mips/mips.opt

@@ -473,6 +473,10 @@ mframe-header-opt
 Target Var(flag_frame_header_optimization) Optimization
 Optimize frame header.
 
+mgrow-frame-downwards
+Target Var(TARGET_FRAME_GROWS_DOWNWARDS) Init(1) Undocumented
+Change the behaviour to grow the frame downwards.
+
 noasmopt
 Driver