From: Eric Botcazou <ebotca...@adacore.com>
The expansion of additive operations for nonbinary modular types implemented
in the front-end and its counterpart in code generators may create branches,
which is not allowed when restriction No_Implicit_Conditionals is in effect.
This changes it to use an explicit Mod operation when the restriction is in
effect, which is assumed not to create such branches.
gcc/ada/
* exp_ch4.adb (Expand_Nonbinary_Modular_Op): Create an explicit Mod
for additive operations if No_Implicit_Conditionals is in effect.
(Expand_Modular_Addition): Likewise.
(Expand_Modular_Subtraction): Likewise.
(Expand_Modular_Op): Always use an unsigned type obtained by calling
Small_Integer_Type_For on the required size.
Tested on x86_64-pc-linux-gnu, committed on master.
---
gcc/ada/exp_ch4.adb | 132 ++++++++++++++++++++++++++------------------
1 file changed, 77 insertions(+), 55 deletions(-)
diff --git a/gcc/ada/exp_ch4.adb b/gcc/ada/exp_ch4.adb
index 95b7765b173..bf90b46249a 100644
--- a/gcc/ada/exp_ch4.adb
+++ b/gcc/ada/exp_ch4.adb
@@ -139,9 +139,10 @@ package body Exp_Ch4 is
-- case of array type arguments.
procedure Expand_Nonbinary_Modular_Op (N : Node_Id);
- -- When generating C code, convert nonbinary modular arithmetic operations
- -- into code that relies on the front-end expansion of operator Mod. No
- -- expansion is performed if N is not a nonbinary modular operand.
+ -- When generating C code or if restriction No_Implicit_Conditionals is in
+ -- effect, convert most nonbinary modular arithmetic operations into code
+ -- that relies on the expansion of an explicit Mod operator. No expansion
+ -- is performed if N is not a nonbinary modular operation.
procedure Expand_Short_Circuit_Operator (N : Node_Id);
-- Common expansion processing for short-circuit boolean operators
@@ -3899,10 +3900,13 @@ package body Exp_Ch4 is
procedure Expand_Modular_Addition is
begin
- -- If this is not the addition of a constant then compute it using
- -- the general rule: (lhs + rhs) mod Modulus
+ -- If this is not the addition of a constant or else restriction
+ -- No_Implicit_Conditionals is in effect, then compute it using
+ -- the general rule: (lhs + rhs) mod Modulus.
- if Nkind (Right_Opnd (N)) /= N_Integer_Literal then
+ if Nkind (Right_Opnd (N)) /= N_Integer_Literal
+ or else Restriction_Active (No_Implicit_Conditionals)
+ then
Expand_Modular_Op;
-- If this is an addition of a constant, convert it to a subtraction
@@ -3921,6 +3925,7 @@ package body Exp_Ch4 is
Cond_Expr : Node_Id;
Then_Expr : Node_Id;
Else_Expr : Node_Id;
+
begin
-- To prevent spurious visibility issues, convert all
-- operands to Standard.Unsigned.
@@ -3966,12 +3971,12 @@ package body Exp_Ch4 is
-- We will convert to another type (not a nonbinary-modulus modular
-- type), evaluate the op in that representation, reduce the result,
-- and convert back to the original type. This means that the
- -- backend does not have to deal with nonbinary-modulus ops.
-
- Op_Expr : constant Node_Id := New_Op_Node (Nkind (N), Loc);
- Mod_Expr : Node_Id;
+ -- back end does not have to deal with nonbinary-modulus ops.
+ Mod_Expr : Node_Id;
+ Op_Expr : Node_Id;
Target_Type : Entity_Id;
+
begin
-- Select a target type that is large enough to avoid spurious
-- intermediate overflow on pre-reduction computation (for
@@ -3979,22 +3984,15 @@ package body Exp_Ch4 is
declare
Required_Size : Uint := RM_Size (Etype (N));
- Use_Unsigned : Boolean := True;
+
begin
case Nkind (N) is
- when N_Op_Add =>
+ when N_Op_Add | N_Op_Subtract =>
-- For example, if modulus is 255 then RM_Size will be 8
-- and the range of possible values (before reduction) will
-- be 0 .. 508; that range requires 9 bits.
Required_Size := Required_Size + 1;
- when N_Op_Subtract =>
- -- For example, if modulus is 255 then RM_Size will be 8
- -- and the range of possible values (before reduction) will
- -- be -254 .. 254; that range requires 9 bits, signed.
- Use_Unsigned := False;
- Required_Size := Required_Size + 1;
-
when N_Op_Multiply =>
-- For example, if modulus is 255 then RM_Size will be 8
-- and the range of possible values (before reduction) will
@@ -4005,37 +4003,15 @@ package body Exp_Ch4 is
null;
end case;
- if Use_Unsigned then
- if Required_Size <= Standard_Short_Short_Integer_Size then
- Target_Type := Standard_Short_Short_Unsigned;
- elsif Required_Size <= Standard_Short_Integer_Size then
- Target_Type := Standard_Short_Unsigned;
- elsif Required_Size <= Standard_Integer_Size then
- Target_Type := Standard_Unsigned;
- else
- pragma Assert (Required_Size <= 64);
- Target_Type := Standard_Unsigned_64;
- end if;
- elsif Required_Size <= 8 then
- Target_Type := Standard_Integer_8;
- elsif Required_Size <= 16 then
- Target_Type := Standard_Integer_16;
- elsif Required_Size <= 32 then
- Target_Type := Standard_Integer_32;
- else
- pragma Assert (Required_Size <= 64);
- Target_Type := Standard_Integer_64;
- end if;
-
+ Target_Type := Small_Integer_Type_For (Required_Size, Uns => True);
pragma Assert (Present (Target_Type));
end;
+ Op_Expr := New_Op_Node (Nkind (N), Loc);
Set_Left_Opnd (Op_Expr,
- Unchecked_Convert_To (Target_Type,
- New_Copy_Tree (Left_Opnd (N))));
+ Unchecked_Convert_To (Target_Type, New_Copy_Tree (Left_Opnd (N))));
Set_Right_Opnd (Op_Expr,
- Unchecked_Convert_To (Target_Type,
- New_Copy_Tree (Right_Opnd (N))));
+ Unchecked_Convert_To (Target_Type, New_Copy_Tree (Right_Opnd (N))));
-- ??? Why do this stuff for some ops and not others?
if Nkind (N) not in N_Op_And | N_Op_Or | N_Op_Xor then
@@ -4064,13 +4040,24 @@ package body Exp_Ch4 is
Force_Evaluation (Op_Expr, Mode => Strict);
end if;
+ -- Unconditionally add the modulus to the result for a subtraction,
+ -- this gets rid of all its peculiarities by cancelling out the
+ -- addition of the binary modulus in the case where the subtraction
+ -- wraps around in Target_Type.
+
+ if Nkind (N) = N_Op_Subtract then
+ Op_Expr :=
+ Make_Op_Add (Loc,
+ Left_Opnd => Op_Expr,
+ Right_Opnd => Make_Integer_Literal (Loc, Modulus (Typ)));
+ end if;
+
Mod_Expr :=
Make_Op_Mod (Loc,
Left_Opnd => Op_Expr,
Right_Opnd => Make_Integer_Literal (Loc, Modulus (Typ)));
- Rewrite (N,
- Unchecked_Convert_To (Typ, Mod_Expr));
+ Rewrite (N, Unchecked_Convert_To (Typ, Mod_Expr));
end Expand_Modular_Op;
--------------------------------
@@ -4079,10 +4066,13 @@ package body Exp_Ch4 is
procedure Expand_Modular_Subtraction is
begin
- -- If this is not the addition of a constant then compute it using
- -- the general rule: (lhs + rhs) mod Modulus
+ -- If this is not the addition of a constant or else restriction
+ -- No_Implicit_Conditionals is in effect, then compute it using
+ -- the general rule: (lhs - rhs) mod Modulus.
- if Nkind (Right_Opnd (N)) /= N_Integer_Literal then
+ if Nkind (Right_Opnd (N)) /= N_Integer_Literal
+ or else Restriction_Active (No_Implicit_Conditionals)
+ then
Expand_Modular_Op;
-- If this is an addition of a constant, convert it to a subtraction
@@ -4101,6 +4091,7 @@ package body Exp_Ch4 is
Cond_Expr : Node_Id;
Then_Expr : Node_Id;
Else_Expr : Node_Id;
+
begin
Cond_Expr :=
Make_Op_Lt (Loc,
@@ -4139,23 +4130,46 @@ package body Exp_Ch4 is
-- Start of processing for Expand_Nonbinary_Modular_Op
begin
- -- No action needed if front-end expansion is not required or if we
- -- have a binary modular operand.
+ -- No action needed if we have a binary modular operand
- if not Expand_Nonbinary_Modular_Ops
- or else not Non_Binary_Modulus (Typ)
- then
+ if not Non_Binary_Modulus (Typ) then
return;
end if;
case Nkind (N) is
when N_Op_Add =>
+ -- No action needed if front-end expansion is not required and
+ -- restriction No_Implicit_Conditionals is not in effect.
+
+ if not Expand_Nonbinary_Modular_Ops
+ and then not Restriction_Active (No_Implicit_Conditionals)
+ then
+ return;
+ end if;
+
Expand_Modular_Addition;
when N_Op_Subtract =>
+ -- No action needed if front-end expansion is not required and
+ -- restriction No_Implicit_Conditionals is not in effect.
+
+ if not Expand_Nonbinary_Modular_Ops
+ and then not Restriction_Active (No_Implicit_Conditionals)
+ then
+ return;
+ end if;
+
Expand_Modular_Subtraction;
when N_Op_Minus =>
+ -- No action needed if front-end expansion is not required and
+ -- restriction No_Implicit_Conditionals is not in effect.
+
+ if not Expand_Nonbinary_Modular_Ops
+ and then not Restriction_Active (No_Implicit_Conditionals)
+ then
+ return;
+ end if;
-- Expand -expr into (0 - expr)
@@ -4166,6 +4180,14 @@ package body Exp_Ch4 is
Analyze_And_Resolve (N, Typ);
when others =>
+ -- No action needed only if front-end expansion is not required
+ -- because we assume that logical and multiplicative operations
+ -- do not involve implicit conditionals.
+
+ if not Expand_Nonbinary_Modular_Ops then
+ return;
+ end if;
+
Expand_Modular_Op;
end case;
--
2.45.1
