[Ada] Crash on right shift operator for signed integers

[Pierre-Marie de Rodat]

This patch fixes an error in the compiler whereby right shift operators
on signed integers cause a compile-time crash when said shift operators
are provided via pragma Provide_Shift_Operators.

Tested on x86_64-pc-linux-gnu, committed on trunk

gcc/ada/

        * doc/gnat_rm/intrinsic_subprograms.rst (Shifts and Rotates):
        Document behavior on negative numbers
        * gnat_rm.texi: Regenerate.
        * sem_eval.adb (Fold_Shift): Set modulus to be based on the RM
        size for non-modular integer types.
diff --git a/gcc/ada/doc/gnat_rm/intrinsic_subprograms.rst b/gcc/ada/doc/gnat_rm/intrinsic_subprograms.rst
--- a/gcc/ada/doc/gnat_rm/intrinsic_subprograms.rst
+++ b/gcc/ada/doc/gnat_rm/intrinsic_subprograms.rst
@@ -217,7 +217,9 @@ The formal parameter names can be anything.
 
 A more convenient way of providing these shift operators is to use
 the Provide_Shift_Operators pragma, which provides the function declarations
-and corresponding pragma Import's for all five shift functions.
+and corresponding pragma Import's for all five shift functions. Note that in
+using these provided shift operations, shifts performed on negative numbers
+will result in modification of the sign bit.
 
 .. _Source_Location:
 


diff --git a/gcc/ada/gnat_rm.texi b/gcc/ada/gnat_rm.texi
--- a/gcc/ada/gnat_rm.texi
+++ b/gcc/ada/gnat_rm.texi
@@ -17862,7 +17862,9 @@ The formal parameter names can be anything.
 
 A more convenient way of providing these shift operators is to use
 the Provide_Shift_Operators pragma, which provides the function declarations
-and corresponding pragma Import's for all five shift functions.
+and corresponding pragma Import's for all five shift functions. Note that in
+using these provided shift operations, shifts performed on negative numbers
+will result in modification of the sign bit.
 
 @node Source_Location,,Shifts and Rotates,Intrinsic Subprograms
 @anchor{gnat_rm/intrinsic_subprograms source-location}@anchor{271}@anchor{gnat_rm/intrinsic_subprograms id13}@anchor{272}


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
@@ -4815,13 +4815,23 @@ package body Sem_Eval is
          if Op = N_Op_Shift_Left then
             Check_Elab_Call;
 
-            --  Fold Shift_Left (X, Y) by computing (X * 2**Y) rem modulus
+            declare
+               Modulus : Uint;
+            begin
+               if Is_Modular_Integer_Type (Typ) then
+                  Modulus := Einfo.Modulus (Typ);
+               else
+                  Modulus := Uint_2 ** RM_Size (Typ);
+               end if;
 
-            Fold_Uint
-              (N,
-               (Expr_Value (Left) * (Uint_2 ** Expr_Value (Right)))
-                 rem Modulus (Typ),
-               Static => Static);
+               --  Fold Shift_Left (X, Y) by computing (X * 2**Y) rem modulus
+
+               Fold_Uint
+                 (N,
+                  (Expr_Value (Left) * (Uint_2 ** Expr_Value (Right)))
+                    rem Modulus,
+                  Static => Static);
+            end;
 
          elsif Op = N_Op_Shift_Right then
             Check_Elab_Call;