Add a stack_check instruction.

This instruction loads a stack limit from a global variable and compares
it to the stack pointer, trapping if the stack has grown beyond the
limit.

Also add a expand_flags transform group containing legalization patterns
for ISAs with CPU flags.

Fixes #234.

cranelift/docs/langref.rst

@@ -511,6 +511,11 @@ instructions before instruction selection::
     v9 = stack_addr ss3, 16
     v1 = load.f64 v9
 
+When Cretonne code is running in a sandbox, it can also be necessary to include
+stack overflow checks in the prologue.
+
+.. autoinst:: stack_check
+
 Global variables
 ----------------
 

cranelift/filetests/isa/intel/legalize-custom.cton

@@ -12,9 +12,8 @@ ebb0(v1: i32):
     trapz v1, user67
     return
     ; check: $ebb0($v1: i32
-    ; nextln: brnz $v1, $(new=$EBB)
-    ; nextln: trap user67
-    ; check: $new:
+    ; nextln: $(f=$V) = ifcmp_imm $v1, 0
+    ; nextln: trapif eq $f, user67
     ; nextln: return
 }
 
@@ -23,9 +22,8 @@ ebb0(v1: i32):
     trapnz v1, int_ovf
     return
     ; check: $ebb0($v1: i32
-    ; nextln: brz $v1, $(new=$EBB)
-    ; nextln: trap int_ovf
-    ; check: $new:
+    ; nextln: $(f=$V) = ifcmp_imm $v1, 0
+    ; nextln: trapif ne $f, int_ovf
     ; nextln: return
 }
 

cranelift/filetests/isa/intel/legalize-memory.cton

@@ -109,3 +109,17 @@ ebb0(v0: i32, v999: i64):
     ; nextln: $v2 = load.f32 $v1+0x7fff_ffff
     return v2
 }
+
+; Stack overflow check.
+; The stack limit is stored in a pointer-sized global variable.
+function %stkchk(i64 vmctx) spiderwasm {
+    gv0 = vmctx+64
+
+ebb0(v0: i64):
+    ; check: $ebb0(
+    stack_check gv0
+    ; check: $(limit=$V) = load.i64 notrap aligned
+    ; check: $(flags=$V) = ifcmp_sp $limit
+    ; check: trapif uge $flags, stk_ovf
+    return
+}