Rename "local variables" to "explicit stack slots".

The term "local variables" predated the SSA builder in the front-end
crate, which also provides a way to implement source-language local
variables. The name "explicit stack slot" makes it clear what this
construct is.

cranelift/docs/example.cton

@@ -1,7 +1,7 @@
 test verifier
 
 function %average(i32, i32) -> f32 native {
-    ss1 = local 8            ; Stack slot for ``sum``.
+    ss1 = explicit_slot 8         ; Stack slot for ``sum``.
 
 ebb1(v1: i32, v2: i32):
     v3 = f64const 0x0.0

cranelift/docs/langref.rst

@@ -37,7 +37,7 @@ The first line of a function definition provides the function *name* and
 the :term:`function signature` which declares the parameter and return types.
 Then follows the :term:`function preamble` which declares a number of entities
 that can be referenced inside the function. In the example above, the preamble
-declares a single local variable, ``ss1``.
+declares a single explicit stack slot, ``ss1``.
 
 After the preamble follows the :term:`function body` which consists of
 :term:`extended basic block`\s (EBBs), the first of which is the
@@ -471,8 +471,8 @@ the expected alignment. By default, misaligned loads and stores are allowed,
 but when the ``aligned`` flag is set, a misaligned memory access is allowed to
 :term:`trap`.
 
-Local variables
----------------
+Explicit Stack Slots
+--------------------
 
 One set of restricted memory operations access the current function's stack
 frame. The stack frame is divided into fixed-size stack slots that are
@@ -480,9 +480,9 @@ allocated in the :term:`function preamble`. Stack slots are not typed, they
 simply represent a contiguous sequence of :term:`accessible` bytes in the stack
 frame.
 
-.. inst:: SS = local Bytes, Flags...
+.. inst:: SS = explicit_slot Bytes, Flags...
 
-    Allocate a stack slot for a local variable in the preamble.
+    Allocate a stack slot in the preamble.
 
     If no alignment is specified, Cretonne will pick an appropriate alignment
     for the stack slot based on its size and access patterns.
@@ -1135,7 +1135,7 @@ Glossary
         A list of declarations of entities that are used by the function body.
         Some of the entities that can be declared in the preamble are:
 
-        - Local variables.
+        - Stack slots.
         - Functions that are called directly.
         - Function signatures for indirect function calls.
         - Function flags and attributes that are not part of the signature.
@@ -1158,9 +1158,16 @@ Glossary
         intermediate representation. Cretonne's IR can be converted to text
         losslessly.
 
-    stack slot
+    explicit stack slot
         A fixed size memory allocation in the current function's activation
-        frame. Also called a local variable.
+        frame. These differ from :term:`spill stack slots` in that they can
+        be created by frontends and they may have their addresses taken.
+
+    spill stack slot
+        A fixed size memory allocation in the current function's activation
+        frame. These differ from :term:`explicit stack slots` in that they are
+        only created during register allocation, and they may not have their
+        address taken.
 
     terminator instruction
         A control flow instruction that unconditionally directs the flow of

cranelift/filetests/isa/intel/prologue-epilogue.cton

@@ -4,13 +4,13 @@ set is_compressed
 isa intel haswell
 
 function %foo() {
-    ss0 = local 168
+    ss0 = explicit_slot 168
 ebb0:
     return
 }
 
 ; check: function %foo(i64 fp [%rbp], i64 csr [%rbx], i64 csr [%r12], i64 csr [%r13], i64 csr [%r14], i64 csr [%r15]) -> i64 fp [%rbp], i64 csr [%rbx], i64 csr [%r12], i64 csr [%r13], i64 csr [%r14], i64 csr [%r15] native {
-; nextln:     ss0 = local 168, offset -224
+; nextln:     ss0 = explicit_slot 168, offset -224
 ; nextln:     ss1 = incoming_arg 56, offset -56
 ; check: ebb0(v0: i64 [%rbp], v1: i64 [%rbx], v2: i64 [%r12], v3: i64 [%r13], v4: i64 [%r14], v5: i64 [%r15]):
 ; nextln:     x86_push v0
@@ -29,4 +29,4 @@ ebb0:
 ; nextln:     v7 = x86_pop.i64
 ; nextln:     v6 = x86_pop.i64
 ; nextln:     return v6, v7, v8, v9, v10, v11
-; nextln: }
+; nextln: }

cranelift/filetests/parser/tiny.cton

@@ -124,7 +124,7 @@ ebb0(v90: i32, v91: f32):
 ; Stack slot references
 function %stack() {
     ss10 = spill_slot 8
-    ss2 = local 4
+    ss2 = explicit_slot 4
     ss3 = incoming_arg 4, offset 8
     ss4 = outgoing_arg 4
     ss5 = emergency_slot 4
@@ -136,7 +136,7 @@ ebb0:
     stack_store v2, ss2
 }
 ; sameln: function %stack() native {
-; check:     ss2 = local 4
+; check:     ss2 = explicit_slot 4
 ; check:     ss3 = incoming_arg 4, offset 8
 ; check:     ss4 = outgoing_arg 4
 ; check:     ss5 = emergency_slot 4