[PATCH 16/16] gcc: Hack up the arguments to the linker

[David Malcolm]

This is a nasty hack, but I noticed a lot of time spent executing the
linker within jit.dg/test-benchmark, with link lines like:

  run_embedded_ld: 24 args
    argv[0]: ld
    argv[1]: --eh-frame-hdr
    argv[2]: -m
    argv[3]: elf_x86_64
    argv[4]: -shared
    argv[5]: -o
    argv[6]: /tmp/libgccjit-mxDkPa/fake.so
    argv[7]: /lib/../lib64/crti.o
    argv[8]: 
/home/david/coding-3/gcc-git-jit-benchmarks/build-jit/gcc/crtbeginS.o
    argv[9]: -L/lib/../lib64
    argv[10]: -L/usr/lib/../lib64
    argv[11]: -L/home/david/coding-3/gcc-git-jit-benchmarks/build-jit/gcc
    argv[12]: /tmp/ccSlluGf.o
    argv[13]: -lgcc
    argv[14]: --as-needed
    argv[15]: -lgcc_s
    argv[16]: --no-as-needed
    argv[17]: -lc
    argv[18]: -lgcc
    argv[19]: --as-needed
    argv[20]: -lgcc_s
    argv[21]: --no-as-needed
    argv[22]: 
/home/david/coding-3/gcc-git-jit-benchmarks/build-jit/gcc/crtendS.o
    argv[23]: /lib/../lib64/crtn.o

Much of this appears to be unnecessary; if I purge the link line to
just:

    argv[0]: ld
    argv[1]: --eh-frame-hdr
    argv[2]: -m
    argv[3]: elf_x86_64
    argv[4]: -shared
    argv[5]: -o
    argv[6]: /tmp/libgccjit-mxDkPa/fake.so
    argv[7]: /tmp/ccSlluGf.o

the benchmark gets a 2x speedup.

Clearly this patch isn't appropriate as-is; when are the various
objects/libraries above that I hacked out actually needed?  Is there
a way programmatically determine what's needed?

gcc/ChangeLog:
        * gcc.c (purge_ld_arg): HACK! Purge the arguments to the
        linker.
        (run_embedded_ld): Likewise.
---
 gcc/gcc.c | 42 ++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 42 insertions(+)

diff --git a/gcc/gcc.c b/gcc/gcc.c
index ed92c7d..dc036e8 100644
--- a/gcc/gcc.c
+++ b/gcc/gcc.c
@@ -2887,6 +2887,25 @@ static int run_embedded_as (int argc, const char **argv)
 
 #ifdef HAVE_LIBLD
 
+/* HACK!  */
+static bool
+purge_ld_arg (const char *arg)
+{
+  /* Hack away "-l", e.g. -lgcc, -lgcc_s, -lc.  */
+  if (0 == strncmp (arg, "-l", 2))
+    return true;
+
+  /* If we can survive without -l, we don't need -L.  */
+  if (0 == strncmp (arg, "-L", 2))
+    return true;
+
+  /* Hack away crti.o, crtbeginS.o, crtendS.o, crtn.o.  */
+  if (strstr (arg, "/crt"))
+    return true;
+
+  return false;
+}
+
 /* Invoke ld_main, passing in the driver's timer
    so that the linker code can record timing information into it.  */
 
@@ -2902,6 +2921,29 @@ static int run_embedded_ld (int argc, const char **argv)
        fprintf (stderr, "  argv[%i]: %s\n", i, argv[i]);
     }
 
+  /* FIXME: Hack.  Purge the linker line, to see if we can speed things
+     up.   */
+  int idx = 0;
+  while (idx < argc)
+    {
+      if (purge_ld_arg (argv[idx]))
+       {
+         memmove (&argv[idx], &argv[idx + 1],
+                  sizeof (char *) * (argc - (idx + 1)));
+         argc--;
+       }
+      else
+       idx++;
+    }
+
+  if (0)
+    {
+      fprintf (stderr, "run_embedded_ld: %i args\n", argc);
+      for (int i = 0; i < argc; i++)
+       fprintf (stderr, "  argv[%i]: %s\n", i, argv[i]);
+    }
+
+
   ctimershim ct (driver_timer, timer::ITEM_DICT_EMBEDDED_LD);
   return ld_main (argc,
                   argv,
-- 
1.8.5.3