Re: [PATCH] AArch64: Cleanup aarch64_classify_symbol

[Wilco Dijkstra <Wilco.Dijkstra@arm.com>]

Hi Richard,

> Just to check: I guess this part is an optimisation, because it
> means that we can share the GOT entry with other TUs.  Is that right?
> I think it would be worth having a comment either way, whatever the
> rationale.  A couple of other very minor things:

It's just to make the code simpler and more orthogonal - the size of the
sequence and sharing of GOT/literal is identical in both cases.

Here is v2 with comments/spaces fixed up:


Use a GOT indirection for extern weak symbols instead of a literal - this is the same as
PIC/PIE and mirrors LLVM behaviour.  Ensure PIC/PIE use the same offset limits for symbols
that don't use the GOT.

Passes bootstrap and regress. OK for commit?

ChangeLog:
2021-04-27  Wilco Dijkstra  <wdijkstr@arm.com>

        * config/aarch64/aarch64.c (aarch64_classify_symbol): Use GOT for extern weak symbols.
        Limit symbol offsets for non-GOT symbols with PIC/PIE.

---

diff --git a/gcc/config/aarch64/aarch64.c b/gcc/config/aarch64/aarch64.c
index 82957dddbe42a7f907b2960294ac7f8abf7be2ff..641c83b479e76cbcc75b299eb7ae5f634d9db7cd 100644
--- a/gcc/config/aarch64/aarch64.c
+++ b/gcc/config/aarch64/aarch64.c
@@ -17854,7 +17854,14 @@ aarch64_classify_symbol (rtx x, HOST_WIDE_INT offset)
 
       switch (aarch64_cmodel)
 	{
+	case AARCH64_CMODEL_TINY_PIC:
 	case AARCH64_CMODEL_TINY:
+	  /* With -fPIC non-local symbols use the GOT.  For orthogonality
+	     always use the GOT for extern weak symbols.  */
+	  if ((flag_pic || SYMBOL_REF_WEAK (x))
+	      && !aarch64_symbol_binds_local_p (x))
+	    return SYMBOL_TINY_GOT;
+
 	  /* When we retrieve symbol + offset address, we have to make sure
 	     the offset does not cause overflow of the final address.  But
 	     we have no way of knowing the address of symbol at compile time
@@ -17862,42 +17869,30 @@ aarch64_classify_symbol (rtx x, HOST_WIDE_INT offset)
 	     symbol + offset is outside the addressible range of +/-1MB in the
 	     TINY code model.  So we limit the maximum offset to +/-64KB and
 	     assume the offset to the symbol is not larger than +/-(1MB - 64KB).
-	     If offset_within_block_p is true we allow larger offsets.
-	     Furthermore force to memory if the symbol is a weak reference to
-	     something that doesn't resolve to a symbol in this module.  */
-
-	  if (SYMBOL_REF_WEAK (x) && !aarch64_symbol_binds_local_p (x))
-	    return SYMBOL_FORCE_TO_MEM;
+	     If offset_within_block_p is true we allow larger offsets.  */
 	  if (!(IN_RANGE (offset, -0x10000, 0x10000)
 		|| offset_within_block_p (x, offset)))
 	    return SYMBOL_FORCE_TO_MEM;
 
 	  return SYMBOL_TINY_ABSOLUTE;
 
+
+	case AARCH64_CMODEL_SMALL_SPIC:
+	case AARCH64_CMODEL_SMALL_PIC:
 	case AARCH64_CMODEL_SMALL:
+	  if ((flag_pic || SYMBOL_REF_WEAK (x))
+	      && !aarch64_symbol_binds_local_p (x))
+	    return aarch64_cmodel == AARCH64_CMODEL_SMALL_SPIC
+		    ? SYMBOL_SMALL_GOT_28K : SYMBOL_SMALL_GOT_4G;
+
 	  /* Same reasoning as the tiny code model, but the offset cap here is
 	     1MB, allowing +/-3.9GB for the offset to the symbol.  */
-
-	  if (SYMBOL_REF_WEAK (x) && !aarch64_symbol_binds_local_p (x))
-	    return SYMBOL_FORCE_TO_MEM;
 	  if (!(IN_RANGE (offset, -0x100000, 0x100000)
 		|| offset_within_block_p (x, offset)))
 	    return SYMBOL_FORCE_TO_MEM;
 
 	  return SYMBOL_SMALL_ABSOLUTE;
 
-	case AARCH64_CMODEL_TINY_PIC:
-	  if (!aarch64_symbol_binds_local_p (x))
-	    return SYMBOL_TINY_GOT;
-	  return SYMBOL_TINY_ABSOLUTE;
-
-	case AARCH64_CMODEL_SMALL_SPIC:
-	case AARCH64_CMODEL_SMALL_PIC:
-	  if (!aarch64_symbol_binds_local_p (x))
-	    return (aarch64_cmodel == AARCH64_CMODEL_SMALL_SPIC
-		    ?  SYMBOL_SMALL_GOT_28K : SYMBOL_SMALL_GOT_4G);
-	  return SYMBOL_SMALL_ABSOLUTE;
-
 	case AARCH64_CMODEL_LARGE:
 	  /* This is alright even in PIC code as the constant
 	     pool reference is always PC relative and within