Index: clang/docs/ThinLTO.rst
===================================================================
--- clang/docs/ThinLTO.rst
+++ clang/docs/ThinLTO.rst
@@ -173,6 +173,10 @@
   ``cache_size_bytes=1g`` on its own will cause both the 1GB and default 75%
   policies to be applied unless the default ``cache_size`` is overridden.
 
+- ``cache_size_files=X``:
+  Set the maximum number of files in the cache directory. Set to 0 to indicate
+  no limit. The default is 1000000 files.
+
 - ``prune_after=Xs``, ``prune_after=Xm``, ``prune_after=Xh``: Sets the
   expiration time for cache files to ``X`` seconds (or minutes, hours
   respectively).  When a file hasn't been accessed for ``prune_after`` seconds,
Index: lld/test/ELF/lto/cache.ll
===================================================================
--- lld/test/ELF/lto/cache.ll
+++ lld/test/ELF/lto/cache.ll
@@ -23,6 +23,14 @@
 ; RUN: ld.lld --thinlto-cache-dir=%t.cache --thinlto-cache-policy cache_size_bytes=32k:prune_interval=0s -o %t3 %t2.o %t.o
 ; RUN: ls %t.cache | count 4
 
+; Setting max number of files to 0 should disable the limit, not delete everything.
+; RUN: ld.lld --thinlto-cache-dir=%t.cache --thinlto-cache-policy cache_size_files=0:prune_interval=0s -o %t3 %t2.o %t.o
+; RUN: ls %t.cache | count 4
+
+; Delete everything except for the timestamp, "foo" and one cache file.
+; RUN: ld.lld --thinlto-cache-dir=%t.cache --thinlto-cache-policy cache_size_files=1:prune_interval=0s -o %t3 %t2.o %t.o
+; RUN: ls %t.cache | count 3
+
 target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
 target triple = "x86_64-unknown-linux-gnu"
 
Index: llvm/include/llvm/Support/CachePruning.h
===================================================================
--- llvm/include/llvm/Support/CachePruning.h
+++ llvm/include/llvm/Support/CachePruning.h
@@ -46,6 +46,15 @@
   /// of available space on the disk will be reduced to the amount of available
   /// space. A value of 0 disables the absolute size-based pruning.
   uint64_t MaxSizeBytes = 0;
+
+  /// The maximum number of files in the cache directory. A value of 0 disables
+  /// the number of files based pruning.
+  ///
+  /// This defaults to 1000000 because with that many files there are
+  /// diminishing returns on the effectiveness of the cache, and some file
+  /// systems have a limit on how many files can be contained in a directory
+  /// (notably ext4, which is limited to around 6000000 files).
+  uint64_t MaxSizeFiles = 1000000;
 };
 
 /// Parse the given string as a cache pruning policy. Defaults are taken from a
Index: llvm/lib/Support/CachePruning.cpp
===================================================================
--- llvm/lib/Support/CachePruning.cpp
+++ llvm/lib/Support/CachePruning.cpp
@@ -113,6 +113,10 @@
         return make_error<StringError>("'" + Value + "' not an integer",
                                        inconvertibleErrorCode());
       Policy.MaxSizeBytes = Size * Mult;
+    } else if (Key == "cache_size_files") {
+      if (Value.getAsInteger(0, Policy.MaxSizeFiles))
+        return make_error<StringError>("'" + Value + "' not an integer",
+                                       inconvertibleErrorCode());
     } else {
       return make_error<StringError>("Unknown key: '" + Key + "'",
                                      inconvertibleErrorCode());
@@ -225,6 +229,26 @@
     FileSizes.insert({StatusOrErr->getSize(), std::string(File->path())});
   }
 
+  auto FileAndSize = FileSizes.rbegin();
+  size_t NumFiles = FileSizes.size();
+
+  auto RemoveCacheFile = [&]() {
+    // Remove the file.
+    sys::fs::remove(FileAndSize->second);
+    // Update size
+    TotalSize -= FileAndSize->first;
+    NumFiles--;
+    DEBUG(dbgs() << " - Remove " << FileAndSize->second << " (size "
+                 << FileAndSize->first << "), new occupancy is " << TotalSize
+                 << "%\n");
+    ++FileAndSize;
+  };
+
+  // Prune for number of files.
+  if (Policy.MaxSizeFiles)
+    while (NumFiles > Policy.MaxSizeFiles)
+      RemoveCacheFile();
+
   // Prune for size now if needed
   if (ShouldComputeSize) {
     auto ErrOrSpaceInfo = sys::fs::disk_space(Path);
@@ -246,18 +270,9 @@
                  << "% target is: " << Policy.MaxSizePercentageOfAvailableSpace
                  << "%, " << Policy.MaxSizeBytes << " bytes\n");
 
-    auto FileAndSize = FileSizes.rbegin();
-    // Remove the oldest accessed files first, till we get below the threshold
-    while (TotalSize > TotalSizeTarget && FileAndSize != FileSizes.rend()) {
-      // Remove the file.
-      sys::fs::remove(FileAndSize->second);
-      // Update size
-      TotalSize -= FileAndSize->first;
-      DEBUG(dbgs() << " - Remove " << FileAndSize->second << " (size "
-                   << FileAndSize->first << "), new occupancy is " << TotalSize
-                   << "%\n");
-      ++FileAndSize;
-    }
+    // Remove the oldest accessed files first, till we get below the threshold.
+    while (TotalSize > TotalSizeTarget && FileAndSize != FileSizes.rend())
+      RemoveCacheFile();
   }
   return true;
 }