From ce91ddca2248fa7d5a73078ccdc822e8669f67fd Mon Sep 17 00:00:00 2001
From: Abul Hossain Khan <140191921+abulgit@users.noreply.github.com>
Date: Thu, 27 Mar 2025 01:02:01 +0530
Subject: [PATCH] Optimize Loc Lookups (#21088)

---
 compiler/src/dmd/location.d | 109 +++++++++++++++++++++++++++++++++++-
 1 file changed, 107 insertions(+), 2 deletions(-)

diff --git a/compiler/src/dmd/location.d b/compiler/src/dmd/location.d
index 7a579b278f..9a396f37e8 100644
--- a/compiler/src/dmd/location.d
+++ b/compiler/src/dmd/location.d
@@ -366,6 +366,11 @@ struct BaseLoc
     uint[] lines; /// For each line, the file offset at which it starts. At index 0 there's always a 0 entry.
     BaseLoc[] substitutions; /// Substitutions from #line / #file directives
 
+    /// Cache for the last line lookup
+    private size_t lastLineIndex = 0;
+    /// Cache for the last substitution lookup
+    private size_t lastSubstIndex = 0;
+
     /// Register that a new line starts at `offset` bytes from the start of the source file
     void newLine(uint offset)
     {
@@ -421,8 +426,61 @@ struct BaseLoc
 
     private size_t getSubstitutionIndex(uint offset) @nogc
     {
+        if (substitutions.length <= 1)
+            return 0;
+
+        // Check if the offset falls within the current cached substitution or the next one
+        if (lastSubstIndex < substitutions.length - 1)
+        {
+            // For the current substitution's range
+            if (substitutions[lastSubstIndex].startIndex <= offset &&
+                (lastSubstIndex == substitutions.length - 1 ||
+                 substitutions[lastSubstIndex + 1].startIndex > offset))
+                return lastSubstIndex;
+
+            // For the next substitution's range
+            if (lastSubstIndex + 1 < substitutions.length - 1 &&
+                substitutions[lastSubstIndex + 1].startIndex <= offset &&
+                substitutions[lastSubstIndex + 2].startIndex > offset)
+            {
+                lastSubstIndex++;
+                return lastSubstIndex;
+            }
+
+            // For the previous substitution's range
+            if (lastSubstIndex > 0 &&
+                substitutions[lastSubstIndex - 1].startIndex <= offset &&
+                substitutions[lastSubstIndex].startIndex > offset)
+            {
+                lastSubstIndex--;
+                return lastSubstIndex;
+            }
+        }
+        else if (lastSubstIndex == substitutions.length - 1 &&
+                 substitutions[lastSubstIndex].startIndex <= offset)
+        {
+            // Last substitution case
+            return lastSubstIndex;
+        }
+
+        // Fall back to binary search, but start near
         size_t lo = 0;
-        size_t hi = substitutions.length + -1;
+        size_t hi = substitutions.length - 1;
+
+        // Adjust the range based on the offset relative to lastSubstIndex
+        if (offset < substitutions[lastSubstIndex].startIndex)
+        {
+            // Search backward
+            lo = 0;
+            hi = lastSubstIndex;
+        }
+        else
+        {
+            // Search forward
+            lo = lastSubstIndex;
+            hi = substitutions.length - 1;
+        }
+
         size_t mid = 0;
         while (lo <= hi)
         {
@@ -430,7 +488,10 @@ struct BaseLoc
             if (substitutions[mid].startIndex <= offset)
             {
                 if (mid == substitutions.length - 1 || substitutions[mid + 1].startIndex > offset)
+                {
+                    lastSubstIndex = mid; // Update cache
                     return mid;
+                }
 
                 lo = mid + 1;
             }
@@ -443,10 +504,51 @@ struct BaseLoc
     }
 
     /// Binary search the index in `this.lines` corresponding to `offset`
+    /// lastLineIndex cache to avoid full binary search when possible
     private size_t getLineIndex(uint offset) @nogc
     {
+        if (lines.length <= 1)
+            return 0;
+
+        if (lastLineIndex < lines.length - 1)
+        {
+            if (lines[lastLineIndex] <= offset && offset < lines[lastLineIndex + 1])
+                return lastLineIndex;
+
+            if (lastLineIndex + 1 < lines.length - 1 &&
+                lines[lastLineIndex + 1] <= offset && offset < lines[lastLineIndex + 2])
+            {
+                lastLineIndex++;
+                return lastLineIndex;
+            }
+
+            if (lastLineIndex > 0 &&
+                lines[lastLineIndex - 1] <= offset && offset < lines[lastLineIndex])
+            {
+                lastLineIndex--;
+                return lastLineIndex;
+            }
+        }
+        else if (lastLineIndex == lines.length - 1 && lines[lastLineIndex] <= offset)
+        {
+            return lastLineIndex;
+        }
+
+        // Fall back to binary search
         size_t lo = 0;
-        size_t hi = lines.length + -1;
+        size_t hi = lines.length - 1;
+
+        if (offset < lines[lastLineIndex])
+        {
+            lo = 0;
+            hi = lastLineIndex;
+        }
+        else
+        {
+            lo = lastLineIndex;
+            hi = lines.length - 1;
+        }
+
         size_t mid = 0;
         while (lo <= hi)
         {
@@ -454,7 +556,10 @@ struct BaseLoc
             if (lines[mid] <= offset)
             {
                 if (mid == lines.length - 1 || lines[mid + 1] > offset)
+                {
+                    lastLineIndex = mid; // Update cache
                     return mid;
+                }
 
                 lo = mid + 1;
             }