incr.comp.: Do less hashing per Span.

3 files changed, 12 insertions, 7 deletions

diff --git a/src/librustc/ich/hcx.rs b/src/librustc/ich/hcx.rs
index 2d20836814c..f9fb668110b 100644
--- a/src/librustc/ich/hcx.rs
+++ b/src/librustc/ich/hcx.rs
@@ -338,13 +338,16 @@ impl<'gcx> HashStable<StableHashingContext<'gcx>> for Span {
             return std_hash::Hash::hash(&TAG_INVALID_SPAN, hasher);
         }
 
-        let len = span.hi - span.lo;
-
         std_hash::Hash::hash(&TAG_VALID_SPAN, hasher);
-        std_hash::Hash::hash(&file_lo.name, hasher);
-        std_hash::Hash::hash(&line_lo, hasher);
-        std_hash::Hash::hash(&col_lo, hasher);
-        std_hash::Hash::hash(&len, hasher);
+        // We truncate the stable_id hash and line and col numbers. The chances
+        // of causing a collision this way should be minimal.
+        std_hash::Hash::hash(&(file_lo.stable_id.0 as u64), hasher);
+
+        let col = (col_lo.0 as u64) & 0xFF;
+        let line = ((line_lo as u64) & 0xFF_FF_FF) << 8;
+        let len = ((span.hi - span.lo).0 as u64) << 32;
+        let line_col_len = col | line | len;
+        std_hash::Hash::hash(&line_col_len, hasher);
 
         if span.ctxt == SyntaxContext::empty() {
             TAG_NO_EXPANSION.hash_stable(hcx, hasher);
diff --git a/src/librustc/ich/impls_syntax.rs b/src/librustc/ich/impls_syntax.rs
index c25aa10eb1e..dfb90a5d27f 100644
--- a/src/librustc/ich/impls_syntax.rs
+++ b/src/librustc/ich/impls_syntax.rs
@@ -394,6 +394,8 @@ impl<'gcx> HashStable<StableHashingContext<'gcx>> for FileMap {
             // Do not hash the source as it is not encoded
             src: _,
             src_hash,
+            // The stable id is just a hash of other fields
+            stable_id: _,
             external_src: _,
             start_pos,
             end_pos: _,
diff --git a/src/librustc/ty/maps/on_disk_cache.rs b/src/librustc/ty/maps/on_disk_cache.rs
index 079b518efd8..7d583a35477 100644
--- a/src/librustc/ty/maps/on_disk_cache.rs
+++ b/src/librustc/ty/maps/on_disk_cache.rs
@@ -176,7 +176,7 @@ impl<'sess> OnDiskCache<'sess> {
                 let index = FileMapIndex(index as u32);
                 let file_ptr: *const FileMap = &**file as *const _;
                 file_to_file_index.insert(file_ptr, index);
-                file_index_to_stable_id.insert(index, StableFilemapId::new(&file));
+                file_index_to_stable_id.insert(index, file.stable_id);
             }
 
             (file_to_file_index, file_index_to_stable_id)