Allow folding constant expressions into single register reads

Essentially all we have to do to make this work is introduce a member of
the Value type, V_attempted_read <reg>, which is returned whenever we
try to read a register value with allow_registers disabled. This defers
the failure from reading the register to the point where the register
value is used (simply because nothing knows how to deal with
V_attempted_read). However, if V_attempted_read is returned directly as
the result of evaluating an expression, then we can replace the
expression with a single direct register read. This optimises some
indirection in the ARM specification.

2 files changed, 31 insertions, 0 deletions

diff --git a/test/c/cfold_reg.expect b/test/c/cfold_reg.expect
new file mode 100644
index 00000000..27ba77dd
--- /dev/null
+++ b/test/c/cfold_reg.expect
@@ -0,0 +1 @@
+true
diff --git a/test/c/cfold_reg.sail b/test/c/cfold_reg.sail
new file mode 100644
index 00000000..a5090e91
--- /dev/null
+++ b/test/c/cfold_reg.sail
@@ -0,0 +1,30 @@
+default Order dec
+
+$include <prelude.sail>
+
+val "eq_string" : (string, string) -> bool
+
+overload operator == = {eq_string}
+
+register R : bool
+
+val "print_endline" : string -> unit
+
+function IMPDEF(str : string) -> bool = {
+  if str == "A" then {
+    return(R)
+  } else if str == "B" then {
+    true
+  } else {
+    false
+  }
+}
+
+function main(() : unit) -> unit = {
+  R = true;
+  if IMPDEF("A") then {
+    print_endline("true")
+  } else {
+    print_endline("false")
+  }
+}
\ No newline at end of file