From 84aa7065deec49bab9fb0b085cd0a0dcc42cc479 Mon Sep 17 00:00:00 2001
From: Nathaniel Shead <nathanieloshead@gmail.com>
Date: Sat, 21 Dec 2024 23:42:28 +1100
Subject: [PATCH] c++/modules: Fallback to ftruncate if posix_fallocate fails
 [PR115008]

Depending on the libc and filesystem, in cases where posix_fallocate
cannot do an efficient preallocation it may return EINVAL.  In such a
case we should fall back to ftruncate instead.

Apparently, depending on the system the use of posix_fallocate can have
a noticeable speedup over ftruncate in general (depending on the system)
so it probably isn't worth it to use ftruncate in all cases.

	PR c++/100358
	PR c++/115008

gcc/cp/ChangeLog:

	* module.cc (elf_out::create_mapping): Fallback to ftruncate if
	posix_fallocate fails.

Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>
---
 gcc/cp/module.cc | 19 ++++++++++++++-----
 1 file changed, 14 insertions(+), 5 deletions(-)

diff --git a/gcc/cp/module.cc b/gcc/cp/module.cc
index b9cf16433b5..9ad587ebd6a 100644
--- a/gcc/cp/module.cc
+++ b/gcc/cp/module.cc
@@ -1905,13 +1905,23 @@ elf_in::begin (location_t loc)
 void
 elf_out::create_mapping (unsigned ext, bool extending)
 {
-#ifndef HAVE_POSIX_FALLOCATE
-#define posix_fallocate(fd,off,len) ftruncate (fd, off + len)
+  /* A wrapper around posix_fallocate, falling back to ftruncate
+     if the underlying filesystem does not support the operation.  */
+  auto allocate = [](int fd, off_t offset, off_t length)
+    {
+#ifdef HAVE_POSIX_FALLOCATE
+      int result = posix_fallocate (fd, offset, length);
+      if (result != EINVAL)
+	return result == 0;
+      /* Not supported by the underlying filesystem, fallback to ftruncate.  */
 #endif
+      return ftruncate (fd, offset + length) == 0;
+    };
+
   void *mapping = MAP_FAILED;
   if (extending && ext < 1024 * 1024)
     {
-      if (!posix_fallocate (fd, offset, ext * 2))
+      if (allocate (fd, offset, ext * 2))
 	mapping = mmap (NULL, ext * 2, PROT_READ | PROT_WRITE,
 			MAP_SHARED, fd, offset);
       if (mapping != MAP_FAILED)
@@ -1919,7 +1929,7 @@ elf_out::create_mapping (unsigned ext, bool extending)
     }
   if (mapping == MAP_FAILED)
     {
-      if (!extending || !posix_fallocate (fd, offset, ext))
+      if (!extending || allocate (fd, offset, ext))
 	mapping = mmap (NULL, ext, PROT_READ | PROT_WRITE,
 			MAP_SHARED, fd, offset);
       if (mapping == MAP_FAILED)
@@ -1929,7 +1939,6 @@ elf_out::create_mapping (unsigned ext, bool extending)
 	  ext = 0;
 	}
     }
-#undef posix_fallocate
   hdr.buffer = (char *)mapping;
   extent = ext;
 }