Re: Thoughts on memcmp expansion (PR43052)

[Nick Clifton <nickc at redhat dot com>]

Hi Bernd,

+      rtx op0 = force_reg (direct_mode, arg1_rtx);
+      rtx op1 = force_reg (direct_mode, arg2_rtx);
+      rtx tem = emit_store_flag (target, NE, op0, op1,
+				 direct_mode, true, false);

This is me being ignorant here... wouldn't it be easier to have a new 
cmpmem_eq pattern (and resulting optab) than to generate this code 
sequence directly ?  That way backends can choose to support this 
optimization, and if they do, they can also choose to support longer 
lengths of comparison.


 DEF_LIB_BUILTIN        (BUILT_IN_MEMCMP, "memcmp", 
BT_FN_INT_CONST_PTR_CONST_PTR_SIZE, ATTR_PURE_NOTHROW_NONNULL_LEAF)
+DEF_GCC_BUILTIN        (BUILT_IN_MEMCMP_EQ, "__memcmp_eq", 
BT_FN_INT_CONST_PTR_CONST_PTR_SIZE, ATTR_PURE_NOTHROW_NONNULL_LEAF)
 DEF_LIB_BUILTIN_CHKP   (BUILT_IN_MEMCPY, "memcpy", 
BT_FN_PTR_PTR_CONST_PTR_SIZE, ATTR_RET1_NOTHROW_NONNULL_LEAF)

Presumably you would also document this new builtin in doc/extend.texi ? 
 Plus maybe add a testcase for it as well ?


+  /* If the return value is used, don't do the transformation.  */

This comment struck me as wrong.  Surely if the return value is not used 
then the entire memcmp can be transformed into nothing.  Plus if the 
return value is used, but only for an equality comparison with zero then 
the transformation can take place.


Cheers
  Nick