* [Ada] Computation of Shift_Left and large signed values
@ 2021-05-07 9:38 Pierre-Marie de Rodat
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From: Pierre-Marie de Rodat @ 2021-05-07 9:38 UTC (permalink / raw)
To: gcc-patches; +Cc: Arnaud Charlet
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The computation of Shift_Left on signed values might wrongly overflow
instead of generating a negative value.
Tested on x86_64-pc-linux-gnu, committed on trunk
gcc/ada/
* sem_eval.adb (Fold_Shift): Fix computation of Shift_Left
resulting in negative signed values.
[-- Attachment #2: patch.diff --]
[-- Type: text/x-diff, Size: 2118 bytes --]
diff --git a/gcc/ada/sem_eval.adb b/gcc/ada/sem_eval.adb
--- a/gcc/ada/sem_eval.adb
+++ b/gcc/ada/sem_eval.adb
@@ -4983,7 +4983,7 @@ package body Sem_Eval is
end if;
end Check_Elab_Call;
- Modulus : Uint;
+ Modulus, Val : Uint;
begin
if Compile_Time_Known_Value (Left)
@@ -4994,23 +4994,25 @@ package body Sem_Eval is
if Op = N_Op_Shift_Left then
Check_Elab_Call;
- declare
- Modulus : Uint;
- begin
- if Is_Modular_Integer_Type (Typ) then
- Modulus := Einfo.Modulus (Typ);
- else
- Modulus := Uint_2 ** RM_Size (Typ);
- end if;
+ if Is_Modular_Integer_Type (Typ) then
+ Modulus := Einfo.Modulus (Typ);
+ else
+ Modulus := Uint_2 ** RM_Size (Typ);
+ end if;
- -- Fold Shift_Left (X, Y) by computing (X * 2**Y) rem modulus
+ -- Fold Shift_Left (X, Y) by computing
+ -- (X * 2**Y) rem modulus [- Modulus]
- Fold_Uint
- (N,
- (Expr_Value (Left) * (Uint_2 ** Expr_Value (Right)))
- rem Modulus,
- Static => Static);
- end;
+ Val := (Expr_Value (Left) * (Uint_2 ** Expr_Value (Right)))
+ rem Modulus;
+
+ if Is_Modular_Integer_Type (Typ)
+ or else Val < Modulus / Uint_2
+ then
+ Fold_Uint (N, Val, Static => Static);
+ else
+ Fold_Uint (N, Val - Modulus, Static => Static);
+ end if;
elsif Op = N_Op_Shift_Right then
Check_Elab_Call;
@@ -5042,7 +5044,7 @@ package body Sem_Eval is
Check_Elab_Call;
declare
- Two_Y : constant Uint := Uint_2 ** Expr_Value (Right);
+ Two_Y : constant Uint := Uint_2 ** Expr_Value (Right);
begin
if Is_Modular_Integer_Type (Typ) then
Modulus := Einfo.Modulus (Typ);
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2021-05-07 9:38 [Ada] Computation of Shift_Left and large signed values Pierre-Marie de Rodat
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