On 10 November 2016 at 15:10, Christophe Lyon
<christophe.l...@linaro.org> wrote:
> On 10 November 2016 at 11:05, Richard Earnshaw
> <richard.earns...@foss.arm.com> wrote:
>> On 09/11/16 21:29, Christophe Lyon wrote:
>>> Hi,
>>> PR 78253 shows that the handling of weak references has changed for
>>> ARM with gcc-5.
>>> When r220674 was committed, default_binds_local_p_2 gained a new
>>> parameter (weak_dominate), which, when true, implies that a reference
>>> to a weak symbol defined locally will be resolved locally, even though
>>> it could be overridden by a strong definition in another object file.
>>> With r220674, default_binds_local_p forces weak_dominate=true,
>>> effectively changing the previous behavior.
>>> The attached patch introduces default_binds_local_p_4 which is a copy
>>> of default_binds_local_p_2, but using weak_dominate=false, and updates
>>> the ARM target to call default_binds_local_p_4 instead of
>>> default_binds_local_p_2.
>>> I ran cross-tests on various arm* configurations with no regression,
>>> and checked that the test attached to the original bugzilla now works
>>> as expected.
>>> I am not sure why weak_dominate defaults to true, and I couldn't
>>> really understand why by reading the threads related to r220674 and
>>> following updates to default_binds_local_p_* which all deal with other
>>> corner cases and do not discuss the weak_dominate parameter.
>>> Or should this patch be made more generic?
>> I certainly don't think it should be ARM specific.
> That was my feeling too.
>> The questions I have are:
>> 1) What do other targets do today.  Are they the same, or different?
> arm, aarch64, s390 use default_binds_local_p_2 since PR 65780, and
> default_binds_local_p before that. Both have weak_dominate=true
> i386 has its own version, calling default_binds_local_p_3 with true
> for weak_dominate
> But the behaviour of default_binds_local_p changed with r220674 as I said 
> above.
> See https://gcc.gnu.org/viewcvs/gcc?view=revision&revision=220674 and
> notice how weak_dominate was introduced
> The original bug report is about a different case:
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=32219
> The original patch submission is
> https://gcc.gnu.org/ml/gcc-patches/2015-02/msg00410.html
> and the 1st version with weak_dominate is in:
> https://gcc.gnu.org/ml/gcc-patches/2015-02/msg00469.html
> but it's not clear to me why this was introduced
>> 2) If different why?
> on aarch64, although binds_local_p returns true, the relocations used when
> building the function pointer is still the same (still via the GOT).
> aarch64 has different logic than arm when accessing a symbol
> (eg aarch64_classify_symbol)
>> 3) Is the current behaviour really what was intended by the patch?  ie.
>> Was the old behaviour actually wrong?
> That's what I was wondering.
> Before r220674, calling a weak function directly or via a function
> pointer had the same effect (in other words, the function pointer
> points to the actual implementation: the strong one if any, the weak
> one otherwise).
> After r220674, on arm the function pointer points to the weak
> definition, which seems wrong to me, it should leave the actual
> resolution to the linker.

After looking at the aarch64 port, I think that references to weak symbols
have to be handled carefully, to make sure they cannot be resolved
by the assembler, since the weak symbol can be overridden by a strong
definition at link-time.

Here is a new patch which does that.
Validated on arm* targets with no regression, and I checked that the
original testcase now executes as expected.


>> R.
>>> Thanks,
>>> Christophe

2016-12-01  Christophe Lyon  <christophe.l...@linaro.org>

        PR target/78253
        * config/arm/arm.c (legitimize_pic_address): Handle reference to
        weak symbol.
        (arm_assemble_integer): Likewise.

diff --git a/gcc/config/arm/arm.c b/gcc/config/arm/arm.c
index 74cb64c..258ceb1 100644
--- a/gcc/config/arm/arm.c
+++ b/gcc/config/arm/arm.c
@@ -6923,10 +6923,13 @@ legitimize_pic_address (rtx orig, machine_mode mode, 
rtx reg)
         same segment as the GOT.  Unfortunately, the flexibility of linker
         scripts means that we can't be sure of that in general, so assume
         that GOTOFF is never valid on VxWorks.  */
+      /* References to weak symbols cannot be resolved locally: they
+        may be overridden by a strong definition at link time.  */
       rtx_insn *insn;
       if ((GET_CODE (orig) == LABEL_REF
-          || (GET_CODE (orig) == SYMBOL_REF &&
-              SYMBOL_REF_LOCAL_P (orig)))
+          || (GET_CODE (orig) == SYMBOL_REF
+              && SYMBOL_REF_LOCAL_P (orig)
+              && (SYMBOL_REF_DECL(orig) ? !DECL_WEAK(SYMBOL_REF_DECL(orig)) : 
          && NEED_GOT_RELOC
          && arm_pic_data_is_text_relative)
        insn = arm_pic_static_addr (orig, reg);
@@ -21583,8 +21586,13 @@ arm_assemble_integer (rtx x, unsigned int size, int 
          /* See legitimize_pic_address for an explanation of the
             TARGET_VXWORKS_RTP check.  */
+         /* References to weak symbols cannot be resolved locally:
+            they may be overridden by a strong definition at link
+            time.  */
          if (!arm_pic_data_is_text_relative
-             || (GET_CODE (x) == SYMBOL_REF && !SYMBOL_REF_LOCAL_P (x)))
+             || (GET_CODE (x) == SYMBOL_REF
+                 && (!SYMBOL_REF_LOCAL_P (x)
+                     || (SYMBOL_REF_DECL(x) ? DECL_WEAK(SYMBOL_REF_DECL(x)) : 
            fputs ("(GOT)", asm_out_file);
            fputs ("(GOTOFF)", asm_out_file);

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