[gcc r11-5345] [Ada] Crash on right shift operator for signed integers

[gcc r11-5345] [Ada] Crash on right shift operator for signed integers

[Pierre-Marie de Rodat]

https://gcc.gnu.org/g:e480bca2400d891bbde62e68d6a8f573848c22c6

commit r11-5345-ge480bca2400d891bbde62e68d6a8f573848c22c6
Author: Justin Squirek <squirek@adacore.com>
Date:   Mon Oct 12 12:06:08 2020 -0400

    [Ada] Crash on right shift operator for signed integers
    
    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:
---
 gcc/ada/doc/gnat_rm/intrinsic_subprograms.rst |  4 +++-
 gcc/ada/gnat_rm.texi                          |  4 +++-
 gcc/ada/sem_eval.adb                          | 22 ++++++++++++++++------
 3 files changed, 22 insertions(+), 8 deletions(-)

diff --git a/gcc/ada/doc/gnat_rm/intrinsic_subprograms.rst b/gcc/ada/doc/gnat_rm/intrinsic_subprograms.rst
index bf9f0b9e6d9..e448816572f 100644
--- 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
index 24afe79993d..decca9ef63f 100644
--- 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
index a04c5fa513c..52aa8c111da 100644
--- 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;