Tighten arm32/arm64 write barrier kill reg sets

The JIT write barrier helpers have a custom calling convention that
avoids killing most registers. The JIT was not taking advantage of
this, and thus was killing unnecessary registers when a write barrier
was necessary. In particular, some integer callee-trash registers
are unaffected by the write barriers, and no floating-point register
is affected.

Also, I got rid of the `FEATURE_WRITE_BARRIER` define, which is always
set. I also put some code under `LEGACY_BACKEND` for easier cleanup
later. I removed some unused defines in target.h for some platforms.

1 files changed, 7 insertions, 6 deletions

diff --git a/src/vm/arm/asmhelpers.asm b/src/vm/arm/asmhelpers.asm
index b2430418d2..60def084e7 100644
--- a/src/vm/arm/asmhelpers.asm
+++ b/src/vm/arm/asmhelpers.asm
@@ -1652,7 +1652,7 @@ DoWrite
 ; over the lifetime of the CLR. Specifically ARM has real problems reading the values of external globals (we
 ; need two memory indirections to do this) so we'd like to be able to directly set the current values of
 ; various GC globals (e.g. g_lowest_address and g_card_table) into the barrier code itself and then reset them
-; every time they change (the GC already calls the VM to inform it of these changes). The handle this without
+; every time they change (the GC already calls the VM to inform it of these changes). To handle this without
 ; creating too much fragility such as hardcoding instruction offsets in the VM update code, we wrap write
 ; barrier creation and GC globals access in a set of macros that create a table of descriptors describing each
 ; offset that must be patched.
@@ -1916,10 +1916,11 @@ pShadow  SETS "r7"
     ; is more important).
     ;
     ;   Input:
-    ;       $ptrReg : register containing the location to be updated
-    ;       $valReg : register containing the value (an objref) to be written to the location above
-    ;       $mp     : boolean indicating whether the code will run on an MP system
-    ;       $tmpReg : additional register that can be trashed (can alias $ptrReg or $valReg if needed)
+    ;       $ptrReg   : register containing the location to be updated
+    ;       $valReg   : register containing the value (an objref) to be written to the location above
+    ;       $mp       : boolean indicating whether the code will run on an MP system
+    ;       $postGrow : boolean: {true} for post-grow version, {false} otherwise
+    ;       $tmpReg   : additional register that can be trashed (can alias $ptrReg or $valReg if needed)
     ;
     ;   Output:
     ;       $tmpReg : trashed (defaults to $ptrReg)
@@ -2005,7 +2006,7 @@ tempReg     SETS "$tmpReg"
 ;
 ; Finally define the write barrier functions themselves. Currently we don't provide variations that use
 ; different input registers. If the JIT wants this at a later stage in order to improve code quality it would
-; be a relatively simply change to implement via an additional macro parameter to WRITE_BARRIER_ENTRY.
+; be a relatively simple change to implement via an additional macro parameter to WRITE_BARRIER_ENTRY.
 ;
 ; The calling convention for the first batch of write barriers is:
 ;