[master] f1c47e486 add v_dont_optimize and enable it for vcl_init / vcl_fini

[Nils Goroll]

commit f1c47e4860f213710bd58f0c41fc1f1437d6c461
Author: Nils Goroll <nils.goroll at uplex.de>
Date:   Mon Mar 2 21:47:47 2020 +0100

    add v_dont_optimize and enable it for vcl_init / vcl_fini
    
    We add an attribute macro which, for gcc, will disable compiler
    optimizations. The intended use are VCL subs which are called exactly
    once such that any additional compilation effort is wasteful with
    respect to the overall vcl.load time.
    
    We also decorate the vcc-generated C code for vcl_init and vcl_fini
    with v_dont_optimize, which is simple and easy:
    
            void v_dont_optimize v_matchproto_(vcl_func_f)
            VGC_function_vcl_fini(VRT_CTX)
    
            void v_dont_optimize v_matchproto_(vcl_func_f)
            VGC_function_vcl_init(VRT_CTX)
    
    A more difficult improvement is left to be done: Any custom vcl subs
    which are called from vcl_init and/or vcl_fini _only_ should also be
    decorated with v_dont_optimize.
    
    With the current code base, determining if a custom sub qualifies
    would require helper code in vcc_xref to check all uses and return the
    usage union.
    
    As #3163 requires a similar mechanism and because we are about to
    enter the pre-6.4 release freeze, the better option seems to be to
    implement this "use mask" when/if #3163 gets in and come back to this
    optimization then.
    
    Closes #3230

diff --git a/include/vdef.h b/include/vdef.h
index 0a60716b0..a9111feb6 100644
--- a/include/vdef.h
+++ b/include/vdef.h
@@ -100,6 +100,12 @@
 #  define v_deprecated_
 #endif
 
+#if __GNUC_PREREQ__(4,4) // added 2008-07-23
+#  define v_dont_optimize __attribute__((optimize("O")))
+#else
+#  define v_dont_optimize
+#endif
+
 /*********************************************************************
  * Pointer alignment magic
  */
diff --git a/lib/libvcc/vcc_compile.c b/lib/libvcc/vcc_compile.c
index b7ccc9cf1..1be668bfd 100644
--- a/lib/libvcc/vcc_compile.c
+++ b/lib/libvcc/vcc_compile.c
@@ -151,7 +151,16 @@ vcc_EmitProc(struct vcc *tl, struct proc *p)
 	AZ(VSB_finish(p->prologue));
 	AZ(VSB_finish(p->body));
 	Fh(tl, 1, "vcl_func_f %s;\n", VSB_data(p->cname));
-	Fc(tl, 1, "\nvoid v_matchproto_(vcl_func_f)\n");
+	/*
+	 * TODO: v_dont_optimize for custom subs called from vcl_init/fini only
+	 *
+	 * Needs infrastructure similar to that in #3163 : custom subs need a
+	 * mask containing the builtin subs calling the custom sub. If that is
+	 * no larger than VCL_MET_TASK_H, we can enable v_dont_optimize
+	 */
+	Fc(tl, 1, "\nvoid %sv_matchproto_(vcl_func_f)\n",
+	   p->method && p->method->bitval & VCL_MET_TASK_H ?
+	   "v_dont_optimize " : "");
 	Fc(tl, 1, "%s(VRT_CTX)\n", VSB_data(p->cname));
 	Fc(tl, 1, "{\n%s\n%s}\n", VSB_data(p->prologue), VSB_data(p->body));
 	VSB_destroy(&p->body);