[PATCH] Significantly improve Thumb code quality when the frame pointer is not eliminated

[Richard Earnshaw]

Conventionally it seems that a frame pointer will point directly at the 
saved registers for a function frame and that any local variables on the 
stack will appear below this (assuming a descending stack model).  That 
means the local variables have a negative offset from the frame pointer.

Unfortunately, Thumb has no memory addressing modes that directly support 
negative offsets from a base pointer, and to make matters worse, 
generating even small negative constants in registers takes at least two 
instructions (load a positive constant and negate it).  So the result is 
that code quality when we need a frame pointer is truly horrendous.

This patch changes the model that we use when we need a frame pointer in 
Thumb code so that we point it at the base of the local variables: these 
then have possitive offsets from the frame pointer and can, in general, be 
accessed using base+offset addressing.  The result is that we can reduce 
code size by as much as one third (yes 33%!) in some non-trivial 
subroutines.

Tested on arm-elf, arm-eabi and bootstrapped (compiling the compiler in 
thumb code) on arm-netbsdelf.

R.

2005-08-20  Richard Earnshaw  <richard.earnshaw@arm.com>

	* arm.h (arm_stack_offsets): Add locals_base field.
	* arm.c (arm_get_frame_offsets): Compute it.
	(thumb_compute_initial_elimination offset): Make the Thumb frame
	pointer point to the base of the local variables.
	(thumb_expand_prologue): Update accordingly.
	(thumb_expand_epilogue): Likewise.

	* arm.md (thumb_movhi_clobber): Make this insn a define_expand.  Change
	mode of clobbered scratch to DImode.  Handle a case that's known to
	need this.


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