[PATCH V2] Emit funcall external declarations only if actually used.

["Jose E. Marchesi" <jose.marchesi@oracle.com>]

[Differences from V1:
- Prototype for call_from_call_insn moved before comment block.
- Reuse the `call' flag for SYMBOL_REF_LIBCALL.
- Fallback to check REG_CALL_DECL in non-direct calls.
- New test to check correct behavior for non-direct calls.]

There are many places in GCC where alternative local sequences are
tried in order to determine what is the cheapest or best alternative
to use in the current target.  When any of these sequences involve a
libcall, the current implementation of emit_library_call_value_1
introduce a side-effect consisting on emitting an external declaration
for the funcall (such as __divdi3) which is thus emitted even if the
sequence that does the libcall is not retained.

This is problematic in targets such as BPF, because the kernel loader
chokes on the spurious symbol __divdi3 and makes the resulting BPF
object unloadable.  Note that BPF objects are not linked before being
loaded.

This patch changes emit_library_call_value_1 to mark the target
SYMBOL_REF as a libcall.  Then, the emission of the external
declaration is done in the first loop of final.cc:shorten_branches.
This happens only if the corresponding sequence has been kept.

Regtested in x86_64-linux-gnu.
Tested with host x86_64-linux-gnu with target bpf-unknown-none.

gcc/ChangeLog

	* rtl.h	(SYMBOL_REF_LIBCALL): Define.
	* calls.cc (emit_library_call_value_1): Do not emit external
	libcall declaration here.
	* final.cc (shorten_branches): Do it here.

gcc/testsuite/ChangeLog

	* gcc.target/bpf/divmod-libcall-1.c: New test.
	* gcc.target/bpf/divmod-libcall-2.c: Likewise.
	* gcc.c-torture/compile/libcall-2.c: Likewise.
---
 gcc/calls.cc                                  |  9 +++---
 gcc/final.cc                                  | 30 +++++++++++++++++++
 gcc/rtl.h                                     |  5 ++++
 .../gcc.c-torture/compile/libcall-2.c         |  8 +++++
 .../gcc.target/bpf/divmod-libcall-1.c         | 19 ++++++++++++
 .../gcc.target/bpf/divmod-libcall-2.c         | 16 ++++++++++
 6 files changed, 83 insertions(+), 4 deletions(-)
 create mode 100644 gcc/testsuite/gcc.c-torture/compile/libcall-2.c
 create mode 100644 gcc/testsuite/gcc.target/bpf/divmod-libcall-1.c
 create mode 100644 gcc/testsuite/gcc.target/bpf/divmod-libcall-2.c

diff --git a/gcc/calls.cc b/gcc/calls.cc
index 1f3a6d5c450..219ea599b16 100644
--- a/gcc/calls.cc
+++ b/gcc/calls.cc
@@ -4388,9 +4388,10 @@ emit_library_call_value_1 (int retval, rtx orgfun, rtx value,
 	|| argvec[i].partial != 0)
       update_stack_alignment_for_call (&argvec[i].locate);
 
-  /* If this machine requires an external definition for library
-     functions, write one out.  */
-  assemble_external_libcall (fun);
+  /* Mark the emitted target as a libcall.  This will be used by final
+     in order to emit an external symbol declaration if the libcall is
+     ever used.  */
+  SYMBOL_REF_LIBCALL (fun) = 1;
 
   original_args_size = args_size;
   args_size.constant = (aligned_upper_bound (args_size.constant
@@ -4735,7 +4736,7 @@ emit_library_call_value_1 (int retval, rtx orgfun, rtx value,
 	       valreg,
 	       old_inhibit_defer_pop + 1, call_fusage, flags, args_so_far);
 
-  if (flag_ipa_ra)
+  if (flag_ipa_ra || SYMBOL_REF_LIBCALL (orgfun))
     {
       rtx datum = orgfun;
       gcc_assert (GET_CODE (datum) == SYMBOL_REF);
diff --git a/gcc/final.cc b/gcc/final.cc
index dd3e22547ac..2041e43fdd1 100644
--- a/gcc/final.cc
+++ b/gcc/final.cc
@@ -804,6 +804,8 @@ make_pass_compute_alignments (gcc::context *ctxt)
 }
 
 \f
+static rtx call_from_call_insn (rtx_call_insn *insn);
+
 /* Make a pass over all insns and compute their actual lengths by shortening
    any branches of variable length if possible.  */
 
@@ -850,6 +852,34 @@ shorten_branches (rtx_insn *first)
   for (insn = get_insns (), i = 1; insn; insn = NEXT_INSN (insn))
     {
       INSN_SHUID (insn) = i++;
+
+      /* If this is a `call' instruction implementing a libcall, and
+         this machine requires an external definition for library
+         functions, write one out.  */
+      if (CALL_P (insn))
+        {
+          rtx x;
+
+          if ((x = call_from_call_insn (dyn_cast <rtx_call_insn *> (insn)))
+              && (x = XEXP (x, 0))
+              && MEM_P (x)
+              && (x = XEXP (x, 0))
+              && SYMBOL_REF_P (x)
+              && SYMBOL_REF_LIBCALL (x))
+            {
+              /* Direct call.  */
+              assemble_external_libcall (x);
+            }
+          else if ((x = find_reg_note (insn, REG_CALL_DECL, NULL_RTX))
+                   && (x = XEXP (x, 0)))
+            {
+              /* Indirect call with REG_CALL_DECL note.  */
+              gcc_assert (SYMBOL_REF_P (x));
+              if (SYMBOL_REF_LIBCALL (x))
+                assemble_external_libcall (x);
+            }
+        }
+
       if (INSN_P (insn))
 	continue;
 
diff --git a/gcc/rtl.h b/gcc/rtl.h
index e1c51156f90..28be708a55f 100644
--- a/gcc/rtl.h
+++ b/gcc/rtl.h
@@ -334,6 +334,7 @@ struct GTY((desc("0"), tag("0"),
      1 in a CALL_INSN logically equivalent to
        ECF_LOOPING_CONST_OR_PURE and DECL_LOOPING_CONST_OR_PURE_P.
      1 in a VALUE is SP_DERIVED_VALUE_P in cselib.cc.
+     1 in a SYMBOL_REF if it is the target of a libcall.
      Dumped as "/c" in RTL dumps.  */
   unsigned int call : 1;
   /* 1 in a REG, MEM, or CONCAT if the value is set at most once, anywhere.
@@ -2734,6 +2735,10 @@ do {								        \
 #define SYMBOL_REF_USED(RTX)						\
   (RTL_FLAG_CHECK1 ("SYMBOL_REF_USED", (RTX), SYMBOL_REF)->used)
 
+/* 1 if RTX is a symbol_ref that represents a libcall target.  */
+#define SYMBOL_REF_LIBCALL(RTX)                                         \
+  (RTL_FLAG_CHECK1 ("SYMBOL_REF_LIBCALL", (RTX), SYMBOL_REF)->call)
+
 /* 1 if RTX is a symbol_ref for a weak symbol.  */
 #define SYMBOL_REF_WEAK(RTX)						\
   (RTL_FLAG_CHECK1 ("SYMBOL_REF_WEAK", (RTX), SYMBOL_REF)->return_val)
diff --git a/gcc/testsuite/gcc.c-torture/compile/libcall-2.c b/gcc/testsuite/gcc.c-torture/compile/libcall-2.c
new file mode 100644
index 00000000000..b33944c83ff
--- /dev/null
+++ b/gcc/testsuite/gcc.c-torture/compile/libcall-2.c
@@ -0,0 +1,8 @@
+/* Make sure that external refences for libcalls are generated even for
+   indirect calls.  */
+
+/* { dg-do compile } */
+/* { dg-options "-O2 -mcmodel=large" { target x86_64-*-* } } */
+/* { dg-final { scan-assembler "globl\t__divti3" } } */
+
+__int128 a, b; void foo () { a = a / b; }
diff --git a/gcc/testsuite/gcc.target/bpf/divmod-libcall-1.c b/gcc/testsuite/gcc.target/bpf/divmod-libcall-1.c
new file mode 100644
index 00000000000..7481076602a
--- /dev/null
+++ b/gcc/testsuite/gcc.target/bpf/divmod-libcall-1.c
@@ -0,0 +1,19 @@
+/* This test makes sure that no spurious external symbol declarations are
+   emitted for libcalls in tried but eventually not used code sequences.  */
+
+/* { dg-do compile } */
+/* { dg-options "-O2 -mcpu=v3" } */
+/* { dg-final { scan-assembler-not "global\t__divdi3" } } */
+/* { dg-final { scan-assembler-not "global\t__moddi3" } } */
+
+int
+foo (unsigned int len)
+{
+  return ((unsigned long)len) * 234 / 5;
+}
+
+int
+bar (unsigned int len)
+{
+  return ((unsigned long)len) * 234 % 5;
+}
diff --git a/gcc/testsuite/gcc.target/bpf/divmod-libcall-2.c b/gcc/testsuite/gcc.target/bpf/divmod-libcall-2.c
new file mode 100644
index 00000000000..792d689395a
--- /dev/null
+++ b/gcc/testsuite/gcc.target/bpf/divmod-libcall-2.c
@@ -0,0 +1,16 @@
+/* { dg-do compile } */
+/* { dg-options "-O2 -mcpu=v3" } */
+/* { dg-final { scan-assembler "global\t__divdi3" } } */
+/* { dg-final { scan-assembler "global\t__moddi3" } } */
+
+int
+foo (unsigned int len)
+{
+  return ((long)len) * 234 / 5;
+}
+
+int
+bar (unsigned int len)
+{
+  return ((long)len) * 234 % 5;
+}
-- 
2.30.2