[PATCH 1/2] Add allow_uninitialized to tree_base.u.bits for VAR_DECL

[YunQiang Su]

From: YunQiang Su <yunqi...@isrc.iscas.ac.cn>

Uninitialized internal temp variable may be useful in some case,
such as for COND_LEN_MUL etc on RISC-V with V extension: If an
const or pre-exists VAR is used, we have to use "undisturbed"
policy; if an uninitialized VAR is used, we can use "agnostic".
With "agnostic", the microarchitectures can omit copying part of
the VAR.

gcc
        * tree-core.h(tree_base): Add u.bits.allow_uninitialized.
        * tree.h: Add new macro TREE_ALLOW_UNINITIALIZED.
        * tree-ssa-uninit.cc(warn_uninit): Don't warn if VAR is
        marked as allow_uninitialized.
---
 gcc/tree-core.h        | 5 ++++-
 gcc/tree-ssa-uninit.cc | 4 ++++
 gcc/tree.h             | 4 ++++
 3 files changed, 12 insertions(+), 1 deletion(-)

diff --git a/gcc/tree-core.h b/gcc/tree-core.h
index 27c569c7702..984201199f6 100644
--- a/gcc/tree-core.h
+++ b/gcc/tree-core.h
@@ -1101,7 +1101,10 @@ struct GTY(()) tree_base {
       unsigned nameless_flag : 1;
       unsigned atomic_flag : 1;
       unsigned unavailable_flag : 1;
-      unsigned spare0 : 2;
+      /* Don't warn if uninitialized.  RISC-V V has tail agnostic/undisturbed
+        policy, which may be get benifits if we use an uninitialized var.  */
+      unsigned allow_uninitialized : 1;
+      unsigned spare0 : 1;
 
       unsigned spare1 : 8;
 
diff --git a/gcc/tree-ssa-uninit.cc b/gcc/tree-ssa-uninit.cc
index 726684e472a..12861e1dbc9 100644
--- a/gcc/tree-ssa-uninit.cc
+++ b/gcc/tree-ssa-uninit.cc
@@ -142,6 +142,10 @@ warn_uninit (opt_code opt, tree t, tree var, gimple 
*context,
   if (!has_undefined_value_p (t))
     return;
 
+  /* VAR may mark itself as allow_uninitialized.  */
+  if (TREE_ALLOW_UNINITIALIZED (var))
+    return;
+
   /* Ignore COMPLEX_EXPR as initializing only a part of a complex
      turns in a COMPLEX_EXPR with the not initialized part being
      set to its previous (undefined) value.  */
diff --git a/gcc/tree.h b/gcc/tree.h
index 28e8e71b036..381780fde2e 100644
--- a/gcc/tree.h
+++ b/gcc/tree.h
@@ -3311,6 +3311,10 @@ extern void decl_fini_priority_insert (tree, 
priority_type);
 #define VAR_DECL_IS_VIRTUAL_OPERAND(NODE) \
   (VAR_DECL_CHECK (NODE)->base.u.bits.saturating_flag)
 
+/* In a VAR_DECL, nonzero if NODE is allowed to be uninitialized.  */
+#define TREE_ALLOW_UNINITIALIZED(NODE) \
+  (VAR_DECL_CHECK (NODE)->base.u.bits.allow_uninitialized)
+
 /* In a VAR_DECL, nonzero if this is a non-local frame structure.  */
 #define DECL_NONLOCAL_FRAME(NODE)  \
   (VAR_DECL_CHECK (NODE)->base.default_def_flag)
-- 
2.45.1