[PATCH 1/2]middle-end: RFC: On expansion of conditional branches, give hint if argument is a truth type to backend

[Tamar Christina via Gcc-patches]

Hi All,

This is an RFC to figure out how to deal with targets that don't have native
comparisons against QImode values.

Booleans, at least in C99 and higher are 0-1 valued.  This means that we only
really need to test a single bit.  However in RTL we no longer have this
information available and just have an SImode value (due to the promotion of
QImode to SImode).

This RFC fixes it by emitting an explicit & 1 during the expansion of the
conditional branch.

However it's unlikely that we want to do this unconditionally.  Most targets
I've tested seem to have harmless code changes, like x86 changes from testb to
andl, $1.

So I have two questions:

1. Should I limit this behind a target macro? Or should I just leave it for all
   targets and deal with the fallout.
2. How can I tell whether the C99 0-1 values bools are being used or the older
   0, non-0 variant?

Bootstrapped Regtested on aarch64-none-linux-gnu and no issues.
However there are some benign codegen changes on x86, testb changed to andl $1.

This pattern occurs more than 120,000 times in SPECCPU 2017 and so is quite 
common.

Thanks,
Tamar

gcc/ChangeLog:

        * tree.h (tree_zero_one_valued_p): New.
        * dojump.cc (do_jump): Add & 1 if truth type.

--- inline copy of patch -- 
diff --git a/gcc/dojump.cc b/gcc/dojump.cc
index 
2af0cd1aca3b6af13d5d8799094ee93f18022296..8eaf1be49cd12298e61c6946ae79ca9de6197864
 100644
--- a/gcc/dojump.cc
+++ b/gcc/dojump.cc
@@ -605,7 +605,17 @@ do_jump (tree exp, rtx_code_label *if_false_label,
       /* Fall through and generate the normal code.  */
     default:
     normal:
-      temp = expand_normal (exp);
+      tree cmp = exp;
+      /* If the expression is a truth type then explicitly generate an & 1
+        to indicate to the target that it's a zero-one values type.  This
+        allows the target to further optimize the comparison should it
+        choose to.  */
+      if (tree_zero_one_valued_p (exp))
+       {
+         type = TREE_TYPE (exp);
+         cmp = build2 (BIT_AND_EXPR, type, exp, build_int_cstu (type, 1));
+       }
+      temp = expand_normal (cmp);
       do_pending_stack_adjust ();
       /* The RTL optimizers prefer comparisons against pseudos.  */
       if (GET_CODE (temp) == SUBREG)
diff --git a/gcc/tree.h b/gcc/tree.h
index 
8f8a9660c9e0605eb516de194640b8c1b531b798..be3d2dee82f692e81082cf21c878c10f9fe9e1f1
 100644
--- a/gcc/tree.h
+++ b/gcc/tree.h
@@ -4690,6 +4690,7 @@ extern tree signed_or_unsigned_type_for (int, tree);
 extern tree signed_type_for (tree);
 extern tree unsigned_type_for (tree);
 extern bool is_truth_type_for (tree, tree);
+extern bool tree_zero_one_valued_p (tree);
 extern tree truth_type_for (tree);
 extern tree build_pointer_type_for_mode (tree, machine_mode, bool);
 extern tree build_pointer_type (tree);




-- 

diff --git a/gcc/dojump.cc b/gcc/dojump.cc
index 
2af0cd1aca3b6af13d5d8799094ee93f18022296..8eaf1be49cd12298e61c6946ae79ca9de6197864
 100644
--- a/gcc/dojump.cc
+++ b/gcc/dojump.cc
@@ -605,7 +605,17 @@ do_jump (tree exp, rtx_code_label *if_false_label,
       /* Fall through and generate the normal code.  */
     default:
     normal:
-      temp = expand_normal (exp);
+      tree cmp = exp;
+      /* If the expression is a truth type then explicitly generate an & 1
+        to indicate to the target that it's a zero-one values type.  This
+        allows the target to further optimize the comparison should it
+        choose to.  */
+      if (tree_zero_one_valued_p (exp))
+       {
+         type = TREE_TYPE (exp);
+         cmp = build2 (BIT_AND_EXPR, type, exp, build_int_cstu (type, 1));
+       }
+      temp = expand_normal (cmp);
       do_pending_stack_adjust ();
       /* The RTL optimizers prefer comparisons against pseudos.  */
       if (GET_CODE (temp) == SUBREG)
diff --git a/gcc/tree.h b/gcc/tree.h
index 
8f8a9660c9e0605eb516de194640b8c1b531b798..be3d2dee82f692e81082cf21c878c10f9fe9e1f1
 100644
--- a/gcc/tree.h
+++ b/gcc/tree.h
@@ -4690,6 +4690,7 @@ extern tree signed_or_unsigned_type_for (int, tree);
 extern tree signed_type_for (tree);
 extern tree unsigned_type_for (tree);
 extern bool is_truth_type_for (tree, tree);
+extern bool tree_zero_one_valued_p (tree);
 extern tree truth_type_for (tree);
 extern tree build_pointer_type_for_mode (tree, machine_mode, bool);
 extern tree build_pointer_type (tree);