From fe12022b68e13f8aea7820edd85ac026e63c3105 Mon Sep 17 00:00:00 2001
From: roblabla <unfiltered@roblab.la>
Date: Tue, 6 Aug 2024 11:38:19 +0200
Subject: [PATCH] Make je_malloc_conf weak on MSVC

Previously, the malloc_conf attribute wasn't as "weak" on MSVC as on
other toolchain. While it is possible to override it in an OBJ file -
in MSVC, every symbol coming from a LIB file can be overriden from an
OBJ file - it isn't possible to override it from another lib file. This
makes it impossible to override it from Rust, for instance, as it always
generates libs and links them together, instead of linking the obj files
directly.

While the weak attribute is not supported by MSVC, it can be emulated
using the /alternatename linker flag. This flag takes two symbols, and
aliases the first symbol to the second if it isn't defined during
linking. This ensures that the default value provided by jemalloc can be
overridden from all contexts, whether the override comes from an obj or
a lib, matching the behavior of gcc's weak attribute.

This is supported by MSVC link.exe since time immemorial, and by
lld-link since 3.7.0.
---
 src/jemalloc.c | 32 ++++++++++++++++++++++++++++++++
 1 file changed, 32 insertions(+)

diff --git a/src/jemalloc.c b/src/jemalloc.c
index abd7540f..cc6a6a23 100644
--- a/src/jemalloc.c
+++ b/src/jemalloc.c
@@ -28,12 +28,31 @@
 /******************************************************************************/
 /* Data. */
 
+#ifdef _MSC_VER
+#define STR(s) #s
+#define XSTR(s) STR(s)
+#endif
+
 /* Runtime configuration options. */
+#ifndef _MSC_VER
 const char	*je_malloc_conf
 #ifndef _WIN32
     JEMALLOC_ATTR(weak)
 #endif
     ;
+#else
+extern const char	*je_malloc_conf;
+const char	*malloc_conf_default = NULL;
+#if defined (_M_IX86)
+// NOTE: The JEMALLOC_PRIVATE_NAMESPACE does not get applied properly on 32-bit
+// windows, presumably due to the leading underscore being automatically added
+// to all symbols.
+#pragma comment(linker, "/alternatename:_" XSTR(je_malloc_conf) "=_malloc_conf_default")
+#else
+#pragma comment(linker, "/alternatename:" XSTR(je_malloc_conf) "=" XSTR(JEMALLOC_PRIVATE_NAMESPACE) "malloc_conf_default")
+#endif
+#endif
+
 /*
  * The usual rule is that the closer to runtime you are, the higher priority
  * your configuration settings are (so the jemalloc config options get lower
@@ -51,11 +70,24 @@ const char	*je_malloc_conf
  * We don't actually want this to be widespread, so we'll give it a silly name
  * and not mention it in headers or documentation.
  */
+#ifndef _MSC_VER
 const char	*je_malloc_conf_2_conf_harder
 #ifndef _WIN32
     JEMALLOC_ATTR(weak)
 #endif
     ;
+#else
+extern const char	*je_malloc_conf_2_conf_harder;
+const char	*malloc_conf_2_conf_harder_default = NULL;
+#if defined (_M_IX86)
+// NOTE: The JEMALLOC_PRIVATE_NAMESPACE does not get applied properly on 32-bit
+// windows, presumably due to the leading underscore being automatically added
+// to all symbols.
+#pragma comment(linker, "/alternatename:_" XSTR(je_malloc_conf_2_conf_harder) "=_malloc_conf_2_conf_harder_default")
+#else
+#pragma comment(linker, "/alternatename:" XSTR(je_malloc_conf_2_conf_harder) "=" XSTR(JEMALLOC_PRIVATE_NAMESPACE) "malloc_conf_2_conf_harder_default")
+#endif
+#endif
 
 const char *opt_malloc_conf_symlink = NULL;
 const char *opt_malloc_conf_env_var = NULL;