https://gcc.gnu.org/g:1ef11f4bed8eb230f04e5fb09741ae6444ca3e7b
commit r15-3401-g1ef11f4bed8eb230f04e5fb09741ae6444ca3e7b
Author: Steve Baird <ba...@adacore.com>
Date: Tue Aug 20 17:35:24 2024 -0700
ada: Transform Length attribute references for non-Strict overflow mode.
The non-strict overflow checking code does a better job of eliminating
overflow checks if given an expression consisting only of predefined
operators (including relationals), literals, identifiers, and conditional
expressions. If it is both feasible and useful, rewrite a
Length attribute reference as such an expression. "Feasible" means
"index type is same type as attribute reference type, so we can rewrite
without
using type conversions". "Useful" means "Overflow_Mode is something other
than
Strict, so there is value in making overflow check elimination easier".
gcc/ada/
* exp_attr.adb (Expand_N_Attribute_Reference): If it makes sense
to do so, then rewrite a Length attribute reference as an
equivalent conditional expression.
Diff:
---
gcc/ada/exp_attr.adb | 69 +++++++++++++++++++++++++++++++++++++++++++++++++++-
1 file changed, 68 insertions(+), 1 deletion(-)
diff --git a/gcc/ada/exp_attr.adb b/gcc/ada/exp_attr.adb
index 84c7a4bbdeeb..702c4bb120a3 100644
--- a/gcc/ada/exp_attr.adb
+++ b/gcc/ada/exp_attr.adb
@@ -4797,7 +4797,7 @@ package body Exp_Attr is
-- then replace this attribute with a reference to 'Range_Length
-- of the appropriate index subtype (since otherwise the
-- back end will try to give us the value of 'Length for
- -- this implementation type).s
+ -- this implementation type).
elsif Is_Constrained (Ptyp) then
Rewrite (N,
@@ -4868,6 +4868,73 @@ package body Exp_Attr is
end if;
end;
+ -- Overflow-related transformations need Length attribute rewritten
+ -- using non-attribute expressions. So generate
+ -- (if Pref'First > Pref'Last
+ -- then 0
+ -- else ((Pref'Last - Pref'First) + 1)) .
+
+ elsif Overflow_Check_Mode in Minimized_Or_Eliminated
+
+ -- This Comes_From_Source test fixes a regression test failure
+ -- involving a Length attribute reference generated as part of
+ -- the expansion of a concatentation operator; it is unclear
+ -- whether this is the right solution to that problem.
+
+ and then Comes_From_Source (N)
+
+ -- This Base_Type equality test is so that we only perform this
+ -- transformation if we can do it without introducing
+ -- a type conversion anywhere in the resulting expansion;
+ -- a type conversion is just as bad as a Length attribute
+ -- reference for those overflow-related transformations.
+
+ and then Btyp = Base_Type (Get_Index_Subtype (N))
+
+ then
+ declare
+ function Prefix_Bound
+ (Bound_Attr_Name : Name_Id; Is_First_Copy : Boolean := False)
+ return Node_Id;
+ -- constructs a Pref'First or Pref'Last attribute reference
+
+ ------------------
+ -- Prefix_Bound --
+ ------------------
+
+ function Prefix_Bound
+ (Bound_Attr_Name : Name_Id; Is_First_Copy : Boolean := False)
+ return Node_Id
+ is
+ Prefix : constant Node_Id :=
+ (if Is_First_Copy
+ then Duplicate_Subexpr (Pref)
+ else Duplicate_Subexpr_No_Checks (Pref));
+ begin
+ return Make_Attribute_Reference (Loc,
+ Prefix => Prefix,
+ Attribute_Name => Bound_Attr_Name,
+ Expressions => New_Copy_List (Exprs));
+ end Prefix_Bound;
+ begin
+ Rewrite (N,
+ Make_If_Expression (Loc,
+ Expressions =>
+ New_List (
+ Node1 => Make_Op_Gt (Loc,
+ Prefix_Bound (Name_First,
+ Is_First_Copy => True),
+ Prefix_Bound (Name_Last)),
+ Node2 => Make_Integer_Literal (Loc, 0),
+ Node3 => Make_Op_Add (Loc,
+ Make_Op_Subtract (Loc,
+ Prefix_Bound (Name_Last),
+ Prefix_Bound (Name_First)),
+ Make_Integer_Literal (Loc, 1)))));
+
+ Analyze_And_Resolve (N, Typ);
+ end;
+
-- Otherwise leave it to the back end
else
