CMD: ignore zero-writes when calculating buffer sizes

chb/app/CHVersion.py

@@ -1,3 +1,3 @@
-chbversion: str = "0.3.0-20260418"
+chbversion: str = "0.3.0-20260527"
 
 minimum_required_chb_version = "0.6.0_20260407"

chb/app/FunctionStackframe.py

@@ -154,6 +154,27 @@ def stackoffset_gap(self, offset: int) -> Optional[int]:
         else:
             return None
 
+    def stackoffset_zero_write_extent(self, offset: int) -> int:
+        """Returns the span from offset upward through consecutive zero-write-only size-4 slots.
+
+        Walks upward (toward less negative offsets) from offset + 4, collecting
+        size-4 slots whose accesses are all zero writes. Stops at the first slot
+        that is absent, has size != 4, has no accesses, or has any non-zero-write
+        access. Returns the total span: (stopping offset) - offset.
+        """
+        current = offset + 4
+        while True:
+            slot = self.stackslot(current)
+            if slot is None or slot.size != 4:
+                break
+            accesses = self.accesses.get(current, [])
+            if not accesses:
+                break
+            if not all(acc.is_zero_write for (_, acc) in accesses):
+                break
+            current += 4
+        return current - offset
+
     @property
     def accesses(self) -> Dict[int, List[Tuple[str, "FnStackAccess"]]]:
         if self._accesses is None:

chb/cmdline/reportcmds.py

@@ -77,6 +77,7 @@
     from chb.app.Function import Function
     from chb.app.FunctionStackframe import FunctionStackframe
     from chb.app.Instruction import Instruction
+    from chb.invariants.FnStackAccess import FnStackAccess
     from chb.invariants.XConstant import XIntConst
     from chb.mips.MIPSInstruction import MIPSInstruction
     from chb.models.BTerm import BTerm, BTermArithmetic
@@ -1262,6 +1263,20 @@ def find_function_attribute(app: "AppAccess", dstarg_index: int, fname: str, ins
 
     return intermediate_attribute
 
+def _has_blockwrite_and_zero_stores_only(
+        accesses: List[Tuple[str, "FnStackAccess"]]) -> bool:
+    """True if accesses contain a block-write and only zero-value stores otherwise."""
+    has_blockwrite = False
+    for (_, acc) in accesses:
+        if acc.is_block_write:
+            has_blockwrite = True
+        elif acc.is_zero_write:
+            pass
+        else:
+            return False
+    return has_blockwrite
+
+
 def calculate_buffer_size(stackframe: "FunctionStackframe",
                           stack_offset: int,
                           instr: "Instruction") -> tuple[Optional[int], str]:
@@ -1302,6 +1317,20 @@ def calculate_buffer_size(stackframe: "FunctionStackframe",
                 + "; replacing it by the size derived from the stacklayout",
                 str(instr), str(stack_offset))
             sizeorigin = "stackframe-layout"
+    elif buffersize == 4:
+        slot_accesses = stackframe.accesses.get(stack_offset, [])
+        if _has_blockwrite_and_zero_stores_only(slot_accesses):
+            buffersize = stackframe.stackoffset_zero_write_extent(stack_offset)
+            if buffersize > 4:
+                chklogger.logger.info(
+                    "Stackbuffer size for %s at offset %s: inferred %s "
+                    + "from zero-initialized slot sequence",
+                    str(instr), str(stack_offset), str(buffersize))
+                sizeorigin = "zero-init-sequence"
+            else:
+                sizeorigin = "stackslot-access"
+        else:
+            sizeorigin = "stackslot-access"
     else:
         sizeorigin = "stackslot-access"
 

chb/invariants/FnStackAccess.py

@@ -90,6 +90,10 @@ def is_block_read(self) -> bool:
     def is_block_write(self) -> bool:
         return False
 
+    @property
+    def is_zero_write(self) -> bool:
+        return False
+
 
 @varregistry.register_tag("rs", FnStackAccess)
 class FnStackRegisterSpill(FnStackAccess):
@@ -251,6 +255,10 @@ def value(self) -> "XXpr":
     def is_store(self) -> bool:
         return True
 
+    @property
+    def is_zero_write(self) -> bool:
+        return self.value.is_int_constant and self.value.is_zero
+
     def __str__(self) -> str:
         return (
             "stack-store("