Core: Give bools C's rules: widen to 0 or 1, narrow by comparing with zero

A bool is ir.IntType(1), so convert() handled it as a 1-bit integer: a
bool slot's plain descriptor read as signed and True sign-extended to
-1 on the way out, and an integer narrowed to a bool by truncation, so
2 became false. Both predate this branch. C widens a bool to 0 or 1 and
narrows to it with != 0, and Python's truthiness agrees on the second.

signedness() now answers unsigned for any 1-bit integer whatever it is
wrapped in, convert() narrows to width 1 with icmp ne 0, and a bool
constant's slot carries the unsigned descriptor like the comparison
results already do.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

Author: r41k0u

pythonbpf/allocation_pass.py

@@ -263,7 +263,7 @@ def _allocate_for_constant(builder, var_name, rval, local_sym_tab):
     """Allocate memory for variable assigned from a constant."""
 
     if isinstance(rval.value, bool):
-        ir_type = ir.IntType(1)
+        ir_type = IntTy(1, False)  # a bool widens to 0 or 1, never sign-extends
         var = builder.alloca(ir_type, name=var_name)
         var.align = 1
         local_sym_tab[var_name] = LocalSymbol(var, ir_type)

pythonbpf/expr/type_normalization.py

@@ -60,6 +60,10 @@ def convert(builder, val, from_ty, to_ty):
             f"integer conversion requested for a {val.type} value to {to_ty}"
         )
     if val.type.width > to_ty.width:
+        if to_ty.width == 1:
+            # C's rule for bool: nonzero is true. Truncation would keep the
+            # low bit and turn 2 into false.
+            return builder.icmp_unsigned("!=", val, ir.Constant(val.type, 0))
         return builder.trunc(val, to_ty)
     if val.type.width < to_ty.width:
         ext = builder.zext if not signedness(from_ty) else builder.sext

pythonbpf/type_deducer.py

@@ -50,6 +50,11 @@ def int_literal_type(value: int) -> IntTy:
 
 
 def signedness(ty) -> bool:
+    """Sign of a descriptor: an IntTy says; a vmlinux Field says through its
+    ctype; a 1-bit integer is a bool and never negative, whatever it is
+    wrapped in; anything else is signed, the pre-signedness default."""
+    if isinstance(ty, ir.IntType) and ty.width == 1:
+        return False
     """Sign of an integer type descriptor.
 
     IntTy carries it directly. A vmlinux Field carries a ctypes class in .type,