[libc] Make the errno macro resolve to the thread local variable directly.

With modern architectures having a thread pointer and language supporting
thread locals, there is no reason to use a function intermediary to access
the thread local errno value.

The entrypoint corresponding to errno has been replaced with an object
library as there is no formal entrypoint for errno anymore.

Reviewed By: jeffbailey, michaelrj

Differential Revision: https://reviews.llvm.org/D120920

Author: Siva Chandra Reddy

libc/config/linux/aarch64/entrypoints.txt

@@ -17,9 +17,6 @@ set(TARGET_LIBC_ENTRYPOINTS
     libc.src.ctype.tolower
     libc.src.ctype.toupper
 
-    # errno.h entrypoints
-    libc.src.errno.__errno_location
-
     # fcntl.h entrypoints
     libc.src.fcntl.creat
     libc.src.fcntl.open

libc/config/linux/api.td

@@ -44,11 +44,8 @@ def NullMacro : MacroDef<"NULL"> {
 
 def ErrnoMacro : MacroDef<"errno"> {
   let Defn = [{
-    #ifdef __cplusplus
-    extern "C"
-    #endif
-    int *__errno_location();
-    #define errno (*__errno_location())
+    extern _Thread_local int __llvmlibc_errno;
+    #define errno __llvmlibc_errno
   }];
 }
 

libc/config/linux/x86_64/entrypoints.txt

@@ -17,9 +17,6 @@ set(TARGET_LIBC_ENTRYPOINTS
     libc.src.ctype.tolower
     libc.src.ctype.toupper
 
-    # errno.h entrypoints
-    libc.src.errno.__errno_location
-
     # fcntl.h entrypoints
     libc.src.fcntl.creat
     libc.src.fcntl.open

libc/include/__llvm-libc-common.h

@@ -29,6 +29,9 @@
 #undef _Alignof
 #define _Alignof alignof
 
+#undef _Thread_local
+#define _Thread_local thread_local
+
 #else // not __cplusplus
 
 #undef __BEGIN_C_DECLS

libc/spec/llvm_libc_ext.td

@@ -33,24 +33,8 @@ def LLVMLibcExt : StandardSpec<"llvm_libc_ext"> {
       ]
   >;
 
-  HeaderSpec Errno = HeaderSpec<
-      "errno.h",
-      [], // Macros
-      [], // Types
-      [], // Enumerations
-      [
-          FunctionSpec<
-              "__errno_location",
-              RetValSpec<IntPtr>,
-              [ArgSpec<VoidType>]
-
-          >,
-      ]
-  >;
-
   let Headers = [
     String,
     Assert,
-    Errno,
   ];
 }

libc/src/__support/CMakeLists.txt

@@ -22,7 +22,7 @@ add_header_library(
   DEPENDS
     .ctype_utils
     libc.include.errno
-    libc.src.errno.__errno_location
+    libc.src.errno.errno
     libc.src.__support.CPP.standalone_cpp
 )
 
@@ -43,7 +43,7 @@ add_header_library(
     .ctype_utils
     .high_precision_decimal
     libc.include.errno
-    libc.src.errno.__errno_location
+    libc.src.errno.errno
     libc.src.__support.CPP.standalone_cpp
     libc.src.__support.FPUtil.fputil
 )

libc/src/__support/FPUtil/CMakeLists.txt

@@ -21,7 +21,7 @@ add_header_library(
     libc.include.fenv
     libc.src.__support.common
     libc.src.__support.CPP.standalone_cpp
-    libc.src.errno.__errno_location
+    libc.src.errno.errno
 )
 
 add_header_library(

libc/src/__support/File/CMakeLists.txt

@@ -5,5 +5,6 @@ add_object_library(
   HDRS
     file.h
   DEPENDS
-    libc.src.errno.__errno_location
+    libc.include.errno
+    libc.src.errno.errno
 )

libc/src/errno/CMakeLists.txt

@@ -1,15 +1,14 @@
 if (LLVM_LIBC_FULL_BUILD)
-add_entrypoint_object(
-  __errno_location
+add_object_library(
+  errno
   SRCS
-    __errno_location.cpp
+    errno.cpp
   HDRS
-    __errno_location.h
     llvmlibc_errno.h
 )
 else()
-add_entrypoint_object(
-  __errno_location
+add_object_library(
+  errno
   SRCS
     dummy_errno.cpp
   HDRS