Stack Limit as an Argument Purpose (#372)

* Initial approach.

* Move stack_limit check before opening the frame

* Account for GPRs and frame pointer in stack check

* Check stack_limit example.

* Remove stack_limit attribute code.

Amends #359

* fmt

cranelift/docs/ir.rst

@@ -546,17 +546,6 @@ instructions before instruction selection::
 When Cranelift code is running in a sandbox, it can also be necessary to include
 stack overflow checks in the prologue.
 
-.. inst:: stack_limit = GV
-
-    Set the stack limit of the current function.
-
-    If set, in the preamble read the stack limit from ``GV`` and compare it to the stack pointer. If
-    the stack pointer has reached or exceeded the limit, generate a trap with a
-    ``stk_ovf`` code.
-
-    Setting `stack_limit` is an alternative way to detect stack overflow, when using
-    a calling convention that doesn't perform stack probes.
-
 Global values
 -------------
 

cranelift/filetests/isa/x86/prologue-epilogue.clif

@@ -228,3 +228,28 @@ ebb4:
 ; check: function %divert
 ; check: regmove v5, %rcx -> %rbx
 ; check: [Op1popq#58,%rbx]                   v15 = x86_pop.i64
+
+; Stack limit checking
+
+function %stack_limit(i64 stack_limit) {
+    ss0 = explicit_slot 168
+ebb0(v0: i64):
+    return
+}
+
+; check: function %stack_limit(i64 stack_limit [%rdi], i64 fp [%rbp]) -> i64 fp [%rbp] fast {
+; nextln:     ss0 = explicit_slot 168, offset -184
+; nextln:     ss1 = incoming_arg 16, offset -16
+; nextln: 
+; nextln: ebb0(v0: i64 [%rdi], v4: i64 [%rbp]):
+; nextln:     v1 = copy v0
+; nextln:     v2 = iadd_imm v1, 16
+; nextln:     v3 = ifcmp_sp v2
+; nextln:     trapif uge v3, stk_ovf
+; nextln:     x86_push v4
+; nextln:     copy_special %rsp -> %rbp
+; nextln:     adjust_sp_down_imm 176
+; nextln:     adjust_sp_up_imm 176
+; nextln:     v5 = x86_pop.i64
+; nextln:     return v5
+; nextln: }

cranelift/filetests/parser/preamble.clif

@@ -1,18 +0,0 @@
-test cat
-
-; Verify parsing of stack_limit.
-function %minimal(i64 vmctx) {
-gv0 = vmctx
-; Stack limit
-stack_limit = gv0
-
-ebb0:
-    trap user0
-}
-; sameln: function %minimal(i64 vmctx) fast {
-; nextln: gv0 = vmctx
-; nextln: stack_limit = gv0
-; nextln: 
-; nextln: ebb0:
-; nextln:     trap user0
-; nextln: }