[gcc r13-2433] LoongArch: add -mdirect-extern-access option

[gcc r13-2433] LoongArch: add -mdirect-extern-access option

[Xi Ruoyao]

https://gcc.gnu.org/g:7742b797b3fd7acea28a0a0bff9bbb06e7c2d0ea

commit r13-2433-g7742b797b3fd7acea28a0a0bff9bbb06e7c2d0ea
Author: Xi Ruoyao <xry111@xry111.site>
Date:   Thu Sep 1 18:38:14 2022 +0800

    LoongArch: add -mdirect-extern-access option
    
    As a new target, LoongArch does not use copy relocation as it's
    problematic in some circumstances.  One bad consequence is we are
    emitting GOT for all accesses to all extern objects with default
    visibility.  The use of GOT is not needed in statically linked
    executables, OS kernels etc.  The GOT entry just wastes space, and the
    GOT access just slow down the execution in those environments.
    
    Before -mexplicit-relocs, we used "-Wa,-mla-global-with-pcrel" to tell
    the assembler not to use GOT for extern access.  But with
    -mexplicit-relocs, we have to opt the logic in GCC.
    
    The name "-mdirect-extern-access" is learnt from x86 port.
    
    gcc/ChangeLog:
    
            * config/loongarch/genopts/loongarch.opt.in: Add
            -mdirect-extern-access option.
            * config/loongarch/loongarch.opt: Regenerate.
            * config/loongarch/loongarch.cc
            (loongarch_symbol_binds_local_p): Return true if
            TARGET_DIRECT_EXTERN_ACCESS.
            (loongarch_option_override_internal): Complain if
            -mdirect-extern-access is used with -fPIC or -fpic.
            * doc/invoke.texi: Document -mdirect-extern-access for
            LoongArch.
    
    gcc/testsuite/ChangeLog:
    
            * gcc.target/loongarch/direct-extern-1.c: New test.
            * gcc.target/loongarch/direct-extern-2.c: New test.

Diff:
---
 gcc/config/loongarch/genopts/loongarch.opt.in        |  4 ++++
 gcc/config/loongarch/loongarch.cc                    |  6 ++++++
 gcc/config/loongarch/loongarch.opt                   |  4 ++++
 gcc/doc/invoke.texi                                  | 15 +++++++++++++++
 gcc/testsuite/gcc.target/loongarch/direct-extern-1.c |  6 ++++++
 gcc/testsuite/gcc.target/loongarch/direct-extern-2.c |  6 ++++++
 6 files changed, 41 insertions(+)

diff --git a/gcc/config/loongarch/genopts/loongarch.opt.in b/gcc/config/loongarch/genopts/loongarch.opt.in
index ebdd9538d48..e10618777b2 100644
--- a/gcc/config/loongarch/genopts/loongarch.opt.in
+++ b/gcc/config/loongarch/genopts/loongarch.opt.in
@@ -184,3 +184,7 @@ Enum(cmodel) String(@@STR_CMODEL_EXTREME@@) Value(CMODEL_EXTREME)
 mcmodel=
 Target RejectNegative Joined Enum(cmodel) Var(la_opt_cmodel) Init(CMODEL_NORMAL)
 Specify the code model.
+
+mdirect-extern-access
+Target Var(TARGET_DIRECT_EXTERN_ACCESS) Init(0)
+Avoid using the GOT to access external symbols.
diff --git a/gcc/config/loongarch/loongarch.cc b/gcc/config/loongarch/loongarch.cc
index 77e3a105390..c9187bf81a7 100644
--- a/gcc/config/loongarch/loongarch.cc
+++ b/gcc/config/loongarch/loongarch.cc
@@ -1610,6 +1610,9 @@ loongarch_weak_symbol_p (const_rtx x)
 bool
 loongarch_symbol_binds_local_p (const_rtx x)
 {
+  if (TARGET_DIRECT_EXTERN_ACCESS)
+    return true;
+
   if (SYMBOL_REF_P (x))
     return (SYMBOL_REF_DECL (x)
 	    ? targetm.binds_local_p (SYMBOL_REF_DECL (x))
@@ -6093,6 +6096,9 @@ loongarch_option_override_internal (struct gcc_options *opts)
   if (loongarch_branch_cost == 0)
     loongarch_branch_cost = loongarch_cost->branch_cost;
 
+  if (TARGET_DIRECT_EXTERN_ACCESS && flag_shlib)
+    error ("%qs cannot be used for compiling a shared library",
+	   "-mdirect-extern-access");
 
   switch (la_target.cmodel)
     {
diff --git a/gcc/config/loongarch/loongarch.opt b/gcc/config/loongarch/loongarch.opt
index 6395234218b..96c811c850b 100644
--- a/gcc/config/loongarch/loongarch.opt
+++ b/gcc/config/loongarch/loongarch.opt
@@ -191,3 +191,7 @@ Enum(cmodel) String(extreme) Value(CMODEL_EXTREME)
 mcmodel=
 Target RejectNegative Joined Enum(cmodel) Var(la_opt_cmodel) Init(CMODEL_NORMAL)
 Specify the code model.
+
+mdirect-extern-access
+Target Var(TARGET_DIRECT_EXTERN_ACCESS) Init(0)
+Avoid using the GOT to access external symbols.
diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
index dd3302fcd15..9d662e35316 100644
--- a/gcc/doc/invoke.texi
+++ b/gcc/doc/invoke.texi
@@ -1017,6 +1017,7 @@ Objective-C and Objective-C++ Dialects}.
 -memcpy  -mno-memcpy -mstrict-align -mno-strict-align @gol
 -mmax-inline-memcpy-size=@var{n} @gol
 -mexplicit-relocs -mno-explicit-relocs @gol
+-mdirect-extern-access -mno-direct-extern-access @gol
 -mcmodel=@var{code-model}}
 
 @emph{M32R/D Options}
@@ -25090,6 +25091,20 @@ GCC build-time by detecting corresponding assembler support:
 @code{-mno-explicit-relocs} otherwise.  This option is mostly useful for
 debugging, or interoperation with assemblers different from the build-time
 one.
+
+@item -mdirect-extern-access
+@itemx -mno-direct-extern-access
+@opindex mdirect-extern-access
+Do not use or use GOT to access external symbols.  The default is
+@option{-mno-direct-extern-access}: GOT is used for external symbols with
+default visibility, but not used for other external symbols.
+
+With @option{-mdirect-extern-access}, GOT is not used and all external
+symbols are PC-relatively addressed.  It is @strong{only} suitable for
+environments where no dynamic link is performed, like firmwares, OS
+kernels, executables linked with @option{-static} or @option{-static-pie}.
+@option{-mdirect-extern-access} is not compatible with @option{-fPIC} or
+@option{-fpic}.
 @end table
 
 @node M32C Options
diff --git a/gcc/testsuite/gcc.target/loongarch/direct-extern-1.c b/gcc/testsuite/gcc.target/loongarch/direct-extern-1.c
new file mode 100644
index 00000000000..85c6c1e8a88
--- /dev/null
+++ b/gcc/testsuite/gcc.target/loongarch/direct-extern-1.c
@@ -0,0 +1,6 @@
+/* { dg-do compile } */
+/* { dg-options "-mexplicit-relocs -mdirect-extern-access" } */
+/* { dg-final { scan-assembler-not "got" } } */
+
+extern int x;
+int f() { return x; }
diff --git a/gcc/testsuite/gcc.target/loongarch/direct-extern-2.c b/gcc/testsuite/gcc.target/loongarch/direct-extern-2.c
new file mode 100644
index 00000000000..58d8bd68a06
--- /dev/null
+++ b/gcc/testsuite/gcc.target/loongarch/direct-extern-2.c
@@ -0,0 +1,6 @@
+/* { dg-do compile } */
+/* { dg-options "-mno-explicit-relocs -mdirect-extern-access" } */
+/* { dg-final { scan-assembler-not "la.global" } } */
+
+extern int x;
+int f() { return x; }