Make GVN aware of instructions that write to CPU flags.

cranelift/filetests/simple_gvn/reject.cton

@@ -23,3 +23,19 @@ ebb0:
     v5 = iadd v4, v3
     return v5
 }
+
+function %cpu_flags() -> b1 {
+ebb0:
+    v0 = iconst.i32 7
+    v1 = iconst.i32 8
+    v2 = ifcmp v0, v1
+    v3 = trueif eq v2
+    v4 = ifcmp v0, v1
+    v5 = trueif eq v4
+    v6 = bor v3, v5
+; check: v2 = ifcmp v0, v1
+; check: v3 = trueif eq v2
+; check: v4 = ifcmp v0, v1
+; check: v5 = trueif eq v4
+    return v6
+}

lib/cretonne/meta/cdsl/instructions.py

@@ -110,6 +110,7 @@ class Instruction(object):
             'can_trap': 'Can this instruction cause a trap?',
             'other_side_effects':
             'Does this instruction have other side effects besides can_*',
+            'writes_cpu_flags': 'Does this instruction write to CPU flags?',
             }
 
     def __init__(self, name, doc, ins=(), outs=(), constraints=(), **kwargs):
@@ -144,6 +145,10 @@ class Instruction(object):
                         "unknown instruction attribute '" + attr + "'")
         for attr in Instruction.ATTRIBS:
             setattr(self, attr, not not kwargs.get(attr, False))
+
+        # Infer the 'writes_cpu_flags' field value.
+        setattr(self, 'writes_cpu_flags', any(out.is_cpu_flags() for out in self.outs))
+
         InstructionGroup.append(self)
 
     def __str__(self):

lib/cretonne/meta/cdsl/operands.py

@@ -242,3 +242,10 @@ class Operand(object):
         dependency.
         """
         return self.kind is not VALUE and self.kind is not VARIABLE_ARGS
+
+    def is_cpu_flags(self):
+        # type: () -> bool
+        """
+        Is this a CPU flags operand?
+        """
+        return self.kind is VALUE and self.typevar.name in ['iflags', 'fflags']

lib/cretonne/src/simple_gvn.rs

@@ -10,7 +10,7 @@ use scoped_hash_map::ScopedHashMap;
 fn trivially_unsafe_for_gvn(opcode: Opcode) -> bool {
     opcode.is_call() || opcode.is_branch() || opcode.is_terminator() ||
         opcode.is_return() || opcode.can_trap() || opcode.other_side_effects() ||
-        opcode.can_store() || opcode.can_load()
+        opcode.can_store() || opcode.can_load() || opcode.writes_cpu_flags()
 }
 
 /// Perform simple GVN on `func`.