Re: [PATCH] Fix DCE REG_LIBCALL note moving from noop move insns (PR rtl-optimization/33644)

[Paolo Bonzini <bonzini at gnu dot org>]

> 1) the old RTL loop optimizer could use them to move loop invariant libcalls.
> Since the old RTL loop optimizer is no more, this is no longer a
> reason to keep the old-style libcall notes.
>
> 2) the old DCE in flow.c could remove entire libcall sequences if the
> libcall result was dead.
> The new DCE uses REG_LIBCALL_ID notes, so again the reason to have
> REG_LIBCALL and REG_RETVAL is gone here.
>
> 3) The pre-regalloc scheduling pass used to move libcall blocks as a whole.

4) When -ftrapv is in effect, libcalls are "necessary" so that the 
results of an operation can be propagated without making the call to the 
libgcc functions dead.  The attached patch causes a failure in pr30286.c.

Paolo
diff --git a/ipa-branch:gcc/optabs.c b/ise:gcc/optabs.c
index 9a7731d..dbff52b 100644
--- a/ipa-branch:gcc/optabs.c
+++ b/ise:gcc/optabs.c
@@ -3425,7 +3373,13 @@ emit_no_conflict_block (rtx insns, rtx target, rtx op0, rtx op1, rtx equiv)
   else
     first = NEXT_INSN (prev);
 
+#if 0
+  /* Encapsulate the block so it gets manipulated as a unit.  */
   maybe_encapsulate_block (first, last, equiv);
+#else
+  (void) first;
+  (void) last;
+#endif
 
   return last;
 }
@@ -3579,7 +3533,13 @@ emit_libcall_block (rtx insns, rtx target, rtx result, rtx equiv)
   else
     first = NEXT_INSN (prev);
 
+#if 0
+  /* Encapsulate the block so it gets manipulated as a unit.  */
   maybe_encapsulate_block (first, last, equiv);
+#else
+  (void) first;
+  (void) last;
+#endif
 }
 
 /* Nonzero if we can perform a comparison of mode MODE straightforwardly.