Index: lld/ELF/LTO.cpp
===================================================================
--- lld/ELF/LTO.cpp
+++ lld/ELF/LTO.cpp
@@ -27,6 +27,7 @@
 #include "llvm/Support/Error.h"
 #include "llvm/Support/FileSystem.h"
 #include "llvm/Support/MemoryBuffer.h"
+#include "llvm/Support/WithColor.h"
 #include <algorithm>
 #include <cstddef>
 #include <memory>
@@ -187,6 +188,38 @@
   return c;
 }
 
+// Check whether cache pruning happens. Add a ThinLTO warning if
+// cache_size_bytes or cache_size_files is too small for the current link job.
+// The warning recommends the user to consider adjusting --thinlto-cache-policy.
+static void
+checkCacheCapacity(const std::vector<std::unique_ptr<MemoryBuffer>> &files,
+                   CachePruningPolicy Policy) {
+  // files.size() is greater the number of inputs  by one. However, a timestamp
+  // file (named llvmcache.timestamp) is created and stored in the cache
+  // directory if --thinlto-cache-policy option is used. llvmcache.timestamp is
+  // used to prune the old cache file. The cache pruning will happen if (the
+  // number of inputs+1) is greater than Policy.MaxSizeFiles. Therefore, I used
+  // files.size() here.
+  if (Policy.MaxSizeFiles && (files.size() > Policy.MaxSizeFiles))
+    WithColor::warning()
+        << "ThinLTO cache pruning happens since the total number of the files "
+           "created by the current link job exceeds the maximum number of "
+           "files; consider adjusting --thinlto-cache-policy.\n";
+
+  if (Policy.MaxSizeBytes > 0) {
+    size_t Size = 0;
+    for (const auto &File : files)
+      if (File)
+        Size += File->getBufferSize();
+
+    if (Size > config->thinLTOCachePolicy.MaxSizeBytes)
+      WithColor::warning()
+          << "ThinLTO cache pruning happens since the total size of the cache "
+             "files consumed by the current link job exceeds maximum cache "
+             "size; consider adjusting --thinlto-cache-policy.\n";
+  }
+}
+
 BitcodeCompiler::BitcodeCompiler() {
   // Initialize indexFile.
   if (!config->thinLTOIndexOnlyArg.empty())
@@ -341,6 +374,9 @@
         },
         cache));
 
+  if (!config->thinLTOCacheDir.empty())
+    checkCacheCapacity(files, config->thinLTOCachePolicy);
+
   // Emit empty index files for non-indexed files but not in single-module mode.
   if (config->thinLTOModulesToCompile.empty()) {
     for (StringRef s : thinIndices) {
Index: lld/test/ELF/lto/cache-warnings.ll
===================================================================
--- /dev/null
+++ lld/test/ELF/lto/cache-warnings.ll
@@ -0,0 +1,39 @@
+; REQUIRES: x86
+; NetBSD: noatime mounts currently inhibit 'touch' from updating atime
+; UNSUPPORTED: system-netbsd
+
+; RUN: opt -module-hash -module-summary %s -o %t.o
+; RUN: opt -module-hash -module-summary %p/Inputs/cache.ll -o %t2.o
+
+; Create two cache files in order to test the warnings.
+; RUN: rm -Rf %t.cache && mkdir %t.cache
+; RUN: touch %t.cache/llvmcache-foo
+; RUN: touch %t.cache/llvmcache-bar
+
+; Check cache policies of the number of files.
+; Case 1: A value of 0 disables the number of files based pruning. Therefore, there is no warning.
+; RUN: ld.lld -v --thinlto-cache-dir=%t.cache --thinlto-cache-policy=prune_interval=0s:cache_size_files=0 %t2.o %t.o -o %t3 2>&1 | FileCheck %s --implicit-check-not=warning
+; Case 2: If the total number of the files created by the current link job is less than the maximum number of files, there is no warning.
+; RUN: ld.lld -v --thinlto-cache-dir=%t.cache --thinlto-cache-policy=prune_interval=0s:cache_size_files=3 %t2.o %t.o -o %t3 2>&1 | FileCheck %s --implicit-check-not=warning
+; Case 3: If the total number of the files created by the current link job exceeds the maximum number of files, a warning is given.
+; RUN: ld.lld --thinlto-cache-dir=%t.cache --thinlto-cache-policy=prune_interval=0s:cache_size_files=1 %t2.o %t.o -o %t3 2>&1 | FileCheck %s --check-prefix=WARN
+
+
+; Check cache policies of the cache size.
+; Case 1: A value of 0 disables the absolute size-based pruning. Therefore, there is no warning.
+; RUN: ld.lld -v --thinlto-cache-dir=%t.cache --thinlto-cache-policy=prune_interval=0s:cache_size_bytes=0 %t2.o %t.o -o %t3 2>&1 | FileCheck %s --implicit-check-not=warning
+; Case 2: If the total size of the cache files created by the current link job is less than the maximum size for the cache directory in bytes, there is no warning.
+; RUN: ld.lld -v --thinlto-cache-dir=%t.cache --thinlto-cache-policy=prune_interval=0s:cache_size_bytes=1355 %t2.o %t.o -o %t3 2>&1 | FileCheck %s --implicit-check-not=warning
+; Case 3: If the total size of the cache files created by the current link job exceeds the maximum size for the cache directory in bytes, a warning is given.
+; RUN: ld.lld --thinlto-cache-dir=%t.cache --thinlto-cache-policy=prune_interval=0s:cache_size_bytes=1350 %t2.o %t.o -o %t3 2>&1 | FileCheck %s --check-prefix=WARN
+
+; WARN: warning: ThinLTO cache pruning happens{{.*}}--thinlto-cache-policy.
+; WARN-NOT: warning: ThinLTO cache pruning happens{{.*}}--thinlto-cache-policy.
+
+target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+define void @globalfunc() {
+entry:
+  ret void
+}