[llvm] [Support] mmap when possible in getSTDIN. (PR #162013)

[via llvm-commits]

https://github.com/aokblast updated https://github.com/llvm/llvm-project/pull/162013

>From ccaa7728ec8d22e49a85762e0cf5908523abfcb3 Mon Sep 17 00:00:00 2001
From: ShengYi Hung <aokblast at FreeBSD.org>
Date: Sun, 5 Oct 2025 21:20:07 +0800
Subject: [PATCH] [Support] mmap when possible in getSTDIN.

When stdin is executed like ./prog < file, we are able to mmap the
buffer so that we can reduce the total memory usage if we try to open a
large file.
---
 llvm/lib/Support/MemoryBuffer.cpp | 26 +++++++++++++++++++++++---
 1 file changed, 23 insertions(+), 3 deletions(-)

diff --git a/llvm/lib/Support/MemoryBuffer.cpp b/llvm/lib/Support/MemoryBuffer.cpp
index 1c4645ad83641..3544a94a04580 100644
--- a/llvm/lib/Support/MemoryBuffer.cpp
+++ b/llvm/lib/Support/MemoryBuffer.cpp
@@ -569,10 +569,30 @@ ErrorOr<std::unique_ptr<MemoryBuffer>> MemoryBuffer::getOpenFileSlice(
 
 ErrorOr<std::unique_ptr<MemoryBuffer>> MemoryBuffer::getSTDIN() {
   // Read in all of the data from stdin, we cannot mmap stdin.
-  //
-  // FIXME: That isn't necessarily true, we should try to mmap stdin and
-  // fallback if it fails.
   sys::ChangeStdinMode(sys::fs::OF_Text);
+  std::error_code EC;
+  sys::fs::file_type Type;
+  sys::fs::file_status Status;
+  EC = sys::fs::status(sys::fs::getStdinHandle(), Status);
+  if (EC)
+    return EC;
+
+  Type = Status.type();
+
+  // If the FD is regular file or block file,
+  // we try to create a mmap buffer first.
+  // If failed, rollback to read and copy.
+  if ((Type == sys::fs::file_type::regular_file ||
+       Type == sys::fs::file_type::block_file) &&
+      shouldUseMmap(sys::fs::getStdinHandle(), Status.getSize(),
+                    Status.getSize(), 0, false,
+                    sys::Process::getPageSizeEstimate(), false)) {
+    std::unique_ptr<MemoryBuffer> Result(
+        new (NamedBufferAlloc("<stdin>")) MemoryBufferMMapFile<MemoryBuffer>(
+            false, sys::fs::getStdinHandle(), Status.getSize(), 0, EC));
+    if (!EC)
+      return std::move(Result);
+  }
 
   return getMemoryBufferForStream(sys::fs::getStdinHandle(), "<stdin>");
 }