rtl-optimization/113255 - base_alias_check vs. pointer difference

When the x86 backend generates code for cpymem with the rep_8byte
strathegy for the 8 byte aligned main rep movq it needs to compute
an adjusted pointer to the source after doing a prologue aligning
the destination.  It computes that via

  src_ptr + (dest_ptr - orig_dest_ptr)

which is perfectly fine.  On RTL this is then

    8: r134:DI=const(`g'+0x44)
    9: {r133:DI=frame:DI-0x4c;clobber flags:CC;}
      REG_UNUSED flags:CC
   56: r129:DI=const(`g'+0x4c)
   57: {r129:DI=r129:DI&0xfffffffffffffff8;clobber flags:CC;}
      REG_UNUSED flags:CC
      REG_EQUAL const(`g'+0x4c)&0xfffffffffffffff8
   58: {r118:DI=r134:DI-r129:DI;clobber flags:CC;}
      REG_DEAD r134:DI
      REG_UNUSED flags:CC
      REG_EQUAL const(`g'+0x44)-r129:DI
   59: {r119:DI=r133:DI-r118:DI;clobber flags:CC;}
      REG_DEAD r133:DI
      REG_UNUSED flags:CC

but as written find_base_term happily picks the first candidate
it finds for the MINUS which means it picks const(`g') rather
than the correct frame:DI.  This way find_base_term (but also
the unfixed find_base_value used by init_alias_analysis to
initialize REG_BASE_VALUE) performs pointer analysis isn't
sound.  The following restricts the handling of multi-operand
operations to the case we know only one can be a pointer.

This for example causes gcc.dg/tree-ssa/pr94969.c to miss some
RTL PRE (I've opened PR113395 for this).  A more drastic patch,
removing base_alias_check results in only gcc.dg/guality/pr41447-1.c
regressing (so testsuite coverage is bad).  I've looked at
gcc.dg/tree-ssa tests and mostly scheduling changes are present,
the cc1plus .text size is only 230 bytes worse.  With the this
less drastic patch below most scheduling changes are gone.

x86_64 might not the very best target to test for impact, but
test coverage on other targets is unlikely to be very much better.

	PR rtl-optimization/113255
	* alias.cc (find_base_term): Remove PLUS/MINUS handling
	when both operands are not CONST_INT_P.

	* gcc.dg/torture/pr113255.c: New testcase.

2 files changed, 32 insertions, 23 deletions

diff --git a/gcc/alias.cc b/gcc/alias.cc
index b2ec480..6fad4b2 100644
--- a/gcc/alias.cc
+++ b/gcc/alias.cc
@@ -2077,31 +2077,13 @@ find_base_term (rtx x, vec<std::pair<cselib_val *,
 	if (tmp1 == pic_offset_table_rtx && CONSTANT_P (tmp2))
 	  return find_base_term (tmp2, visited_vals);
 
-	/* If either operand is known to be a pointer, then prefer it
-	   to determine the base term.  */
-	if (REG_P (tmp1) && REG_POINTER (tmp1))
-	  ;
-	else if (REG_P (tmp2) && REG_POINTER (tmp2))
-	  std::swap (tmp1, tmp2);
-	/* If second argument is constant which has base term, prefer it
-	   over variable tmp1.  See PR64025.  */
-	else if (CONSTANT_P (tmp2) && !CONST_INT_P (tmp2))
+	if (CONST_INT_P (tmp1))
 	  std::swap (tmp1, tmp2);
 
-	/* Go ahead and find the base term for both operands.  If either base
-	   term is from a pointer or is a named object or a special address
-	   (like an argument or stack reference), then use it for the
-	   base term.  */
-	rtx base = find_base_term (tmp1, visited_vals);
-	if (base != NULL_RTX
-	    && ((REG_P (tmp1) && REG_POINTER (tmp1))
-		 || known_base_value_p (base)))
-	  return base;
-	base = find_base_term (tmp2, visited_vals);
-	if (base != NULL_RTX
-	    && ((REG_P (tmp2) && REG_POINTER (tmp2))
-		 || known_base_value_p (base)))
-	  return base;
+	/* We can only handle binary operators when one of the operands
+	   never leads to a base value.  */
+	if (CONST_INT_P (tmp2))
+	  return find_base_term (tmp1, visited_vals);
 
 	/* We could not determine which of the two operands was the
 	   base register and which was the index.  So we can determine
diff --git a/gcc/testsuite/gcc.dg/torture/pr113255.c b/gcc/testsuite/gcc.dg/torture/pr113255.c
new file mode 100644
index 0000000..2f00952
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/torture/pr113255.c
@@ -0,0 +1,27 @@
+/* { dg-do run } */
+/* { dg-additional-options "-mtune=k8 -mstringop-strategy=rep_8byte" { target { x86_64-*-* i?86-*-* } } } */
+
+struct S { unsigned a[10]; unsigned y; unsigned b[6]; } g[2];
+
+__attribute__((noinline, noclone)) int
+test (int x)
+{
+  struct S e[2] = { g[0], g[1] };
+  int r = 0;
+  if (x >= 0)
+    {
+      r++;
+      e[1].y++;
+    }
+  g[1] = e[1];
+  return r;
+}
+
+int
+main ()
+{
+  test (1);
+  if (g[1].y != 1)
+    __builtin_abort ();
+  return 0;
+}