[MaskRay]

https://reviews.llvm.org/D116366 added dontNeedIfMmap for lld/ELF LTO
memory saving (used on read-only maps) https://reviews.llvm.org/D116367.
D119856 (b66b324) switched to posix_madvise to fix the Illumos
build, which made dontNeedIfMmap a no-op on Linux, both glibc and
musl.

(glibc and musl return 0 for POSIX_MADV_DONTNEED without calling
madvise, because POSIX requires posix_madvise not to change the
semantics of memory access, while Linux's MADV_DONTNEED discards data
(which is acceptable for the read-only map use case).

Other systems treat MADV_DONTNEED as a hint. On macOS 26, a 512 MiB
read-only file mapping stays resident after madvise or posix_madvise;
only the next access is slower.

Call madvise on Linux only, restoring the RSS reduction. Make
dontNeedIfMmap a no-op elsewhere.

[llvmorg-github-actions]

@llvm/pr-subscribers-llvm-support

Author: Fangrui Song (MaskRay)

Changes

https://reviews.llvm.org/D116366 added dontNeedIfMmap for lld/ELF LTO
memory saving (used on read-only maps) https://reviews.llvm.org/D116367.
D119856 (b66b324) switched to posix_madvise to fix the Illumos
build, which made dontNeedIfMmap a no-op on Linux, both glibc and
musl.

(glibc and musl return 0 for POSIX_MADV_DONTNEED without calling
madvise, because POSIX requires posix_madvise not to change the
semantics of memory access, while Linux's MADV_DONTNEED discards data
(which is acceptable for the read-only map use case).

Other systems treat MADV_DONTNEED as a hint. On macOS 26, a 512 MiB
read-only file mapping stays resident after madvise or posix_madvise;
only the next access is slower.

Call madvise on Linux only, restoring the RSS reduction. Make
dontNeedIfMmap a no-op elsewhere.

---

Full diff: https://github.com/llvm/llvm-project/pull/240071.diff

2 Files Affected:

• (modified) llvm/include/llvm/Support/MemoryBuffer.h (+1-1)
• (modified) llvm/lib/Support/Unix/Path.inc (+4-5)

diff --git a/llvm/include/llvm/Support/MemoryBuffer.h b/llvm/include/llvm/Support/MemoryBuffer.h
index ea911681bd7760..2811eed86b7c01 100644
--- a/llvm/include/llvm/Support/MemoryBuffer.h
+++ b/llvm/include/llvm/Support/MemoryBuffer.h
@@ -69,7 +69,7 @@ class LLVM_ABI MemoryBuffer {
   /// For read-only MemoryBuffer_MMap, mark the buffer as unused in the near
   /// future and the kernel can free resources associated with it. Further
   /// access is supported but may be expensive. This calls
-  /// madvise(MADV_DONTNEED) on read-only file mappings on *NIX systems. This
+  /// madvise(MADV_DONTNEED) on read-only file mappings on Linux. This
   /// function should not be called on a writable buffer.
   virtual void dontNeedIfMmap() {}
 
diff --git a/llvm/lib/Support/Unix/Path.inc b/llvm/lib/Support/Unix/Path.inc
index 202f7338ca6fa5..5fee0a494f6fd3 100644
--- a/llvm/lib/Support/Unix/Path.inc
+++ b/llvm/lib/Support/Unix/Path.inc
@@ -924,11 +924,10 @@ void mapped_file_region::dontNeedImpl() {
   assert(Mode == mapped_file_region::readonly);
   if (!Mapping)
     return;
-#if defined(__MVS__) || defined(_AIX)
-    // If we don't have madvise, or it isn't beneficial, treat this as a no-op.
-#elif defined(POSIX_MADV_DONTNEED)
-  ::posix_madvise(Mapping, Size, POSIX_MADV_DONTNEED);
-#else
+#ifdef __linux__
+  // Linux's MADV_DONTNEED unmaps the pages and reduces RSS. Other systems
+  // treat it as a hint (macOS keeps the pages resident), and glibc and musl's
+  // posix_madvise ignore POSIX_MADV_DONTNEED.
   ::madvise(Mapping, Size, MADV_DONTNEED);
 #endif
 }