fix: unify CFA offset saturation

Use saturating addition consistently when merging CFA offsets in
DW_OP_fbreg fast paths and EvaluationResult::merge_with_cfa.

Add regression tests for both code paths.

Author: swananan

ghostscope-dwarf/src/core/evaluation.rs

@@ -235,7 +235,7 @@ impl EvaluationResult {
                 // CFA gives us the frame base, add the frame_offset to get final location
                 EvaluationResult::MemoryLocation(LocationResult::RegisterAddress {
                     register,
-                    offset: Some(offset + frame_offset),
+                    offset: Some(offset.saturating_add(frame_offset)),
                     size: None,
                 })
             }
@@ -678,3 +678,28 @@ impl fmt::Display for ComputeStep {
         }
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::{CfaResult, EvaluationResult, LocationResult};
+
+    #[test]
+    fn merge_with_cfa_saturates_register_plus_offset() {
+        let merged = EvaluationResult::Optimized.merge_with_cfa(
+            CfaResult::RegisterPlusOffset {
+                register: 7,
+                offset: i64::MAX - 2,
+            },
+            10,
+        );
+
+        assert_eq!(
+            merged,
+            EvaluationResult::MemoryLocation(LocationResult::RegisterAddress {
+                register: 7,
+                offset: Some(i64::MAX),
+                size: None,
+            })
+        );
+    }
+}

ghostscope-dwarf/src/parser/expression_evaluator.rs

@@ -685,7 +685,7 @@ impl ExpressionEvaluator {
                             } => Ok(EvaluationResult::MemoryLocation(
                                 LocationResult::RegisterAddress {
                                     register,
-                                    offset: Some(cfa_offset + offset),
+                                    offset: Some(cfa_offset.saturating_add(*offset)),
                                     size: None,
                                 },
                             )),
@@ -1604,15 +1604,15 @@ mod tests {
     use super::ExpressionEvaluator;
     use crate::binary::{dwarf_reader_from_arc, DwarfReader};
     use crate::core::{
-        ComputeStep, DirectValueResult, EntryValueCase, EvaluationResult, LocationResult,
+        CfaResult, ComputeStep, DirectValueResult, EntryValueCase, EvaluationResult, LocationResult,
     };
     use crate::index::{BlockNode, CallSiteParameter, CallSiteRecord, FunctionBlocks};
     use gimli::constants;
     use gimli::write::{
         Address, AttributeValue as WriteAttributeValue, Dwarf as WriteDwarf, EndianVec,
         Expression as WriteExpression, LineProgram, Sections, Unit,
     };
-    use gimli::{Format, LittleEndian, Register};
+    use gimli::{Format, LittleEndian, Register, RunTimeEndian};
     use std::sync::Arc;
 
     fn build_scanned_incoming_entry_value_fixture(
@@ -1956,6 +1956,43 @@ mod tests {
         );
     }
 
+    #[test]
+    fn single_fbreg_fast_path_saturates_cfa_offset_addition() {
+        let encoding = gimli::Encoding {
+            format: gimli::Format::Dwarf32,
+            version: 5,
+            address_size: 8,
+        };
+        let get_cfa = |_address| {
+            Ok(Some(CfaResult::RegisterPlusOffset {
+                register: 7,
+                offset: i64::MAX - 3,
+            }))
+        };
+
+        let result = ExpressionEvaluator::parse_expression_with_context(
+            &[0x91, 0x0a],
+            RunTimeEndian::Little,
+            encoding,
+            None,
+            0,
+            Some(&get_cfa),
+            None,
+            None,
+            0,
+        )
+        .expect("single DW_OP_fbreg should parse");
+
+        assert_eq!(
+            result,
+            EvaluationResult::MemoryLocation(LocationResult::RegisterAddress {
+                register: 7,
+                offset: Some(i64::MAX),
+                size: None,
+            })
+        );
+    }
+
     #[test]
     fn big_endian_dw_op_addr_preserves_absolute_address() {
         let encoding = gimli::Encoding {