[clang] Allow CLANG_MODULE_CACHE_PATH env var to override module caching behavior

[abrachet]

Summary

CLANG_MODULE_CACHE_PATH can be used to change where clang should put the module cache, or can be set to "" to disable caching entirely.

[phosek]

To give a bit of context behind this change, we have noticed that as of D125331, a single run of libc++ test suites generates 3.1G of data inside Clang's module cache. This is problematic in CI where we quickly run out of disk space as a consequence. We would like to have a way to either change the cache path to a temporary location, or disable it altogether, that could be used without having to pass an extra flag to every compiler invocation.

[phosek]

clang/test/Driver/modules-cache-path.m

4

This needs a trailing slash.

8

This needs a trailing slash.

[abrachet]

clang/test/Driver/modules-cache-path.m

4

The classic "harmless" cosmetic change that I don't need to run tests again for :)

[phosek]

LGTM this would be really valuable for our use cases, but please wait a few more days to give other reviewers a chance to respond.

clang/lib/Driver/ToolChains/Clang.cpp

static void RenderBuiltinOptions(const ToolChain &TC, const llvm::Triple &T,
   // le32-specific flags:
   //  -fno-math-builtin: clang should not convert math builtins to intrinsics
   //                     by default.
   if (TC.getArch() == llvm::Triple::le32)
     CmdArgs.push_back("-fno-math-builtin");
 }
 
 bool Driver::getDefaultModuleCachePath(SmallVectorImpl<char> &Result) {
+  if (const char *Str = std::getenv("CLANG_MODULE_CACHE_PATH")) {
+    Twine Path{Str};
+    Path.toVector(Result);
+    return Path.getSingleStringRef() != "";
+  }
   if (llvm::sys::path::cache_directory(Result)) {
     llvm::sys::path::append(Result, "clang");
     llvm::sys::path::append(Result, "ModuleCache");
     return true;
   }
   return false;
 }
 

clang/test/Driver/modules-cache-path.m

 // RUN: %clang -fmodules -### %s 2>&1 | FileCheck %s -check-prefix=CHECK-DEFAULT
 // CHECK-DEFAULT: -fmodules-cache-path={{.*}}clang{{[/\\]+}}ModuleCache
+
+// RUN: env CLANG_MODULE_CACHE_PATH=/dev/null \
+// RUN:   %clang -fmodules -### %s 2>&1 | FileCheck %s -check-prefix=OVERRIDE
+// OVERRIDE: -fmodules-cache-path=/dev/null
+
+// RUN: env CLANG_MODULE_CACHE_PATH= \
+// RUN:   %clang -fmodules -### %s 2>&1 | FileCheck %s -check-prefix=DISABLE
+// DISABLE-NOT: -fmodules-cache-path=