[Issue 18795] New: Add pragma(stackStomp, [true|false]) to control -gx on per-function basis

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=18795

          Issue ID: 18795
           Summary: Add pragma(stackStomp, [true|false]) to control -gx on
                    per-function basis
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P1
         Component: dmd
          Assignee: nobody at puremagic.com
          Reporter: bugzilla at digitalmars.com

Stomping the stack is important for detecting certain kinds of dangling
reference bugs (shouldn't be possible in @safe code), but also for
cryptographic code that aims to prevent a function from leaking any information
to its caller:

  http://www.cl.cam.ac.uk/~rja14/Papers/whatyouc.pdf

This should be controllable on a per-function basis, and the best way is
analogous to the way the `pragma(inline)` works, i.e.:

  pragma(stackStomp) // default, meaning set to the setting of the -gx switch
  pragma(stackStomp, true)  // enable for this function
  pragma(stackStomp, false)  // disable for this function

References:

  https://dlang.org/dmd-windows.html#switch-gx
  https://dlang.org/spec/pragma.html#inline
  https://github.com/dlang/dmd/blob/master/src/dmd/backend/cod3.c#L3919

The stomp code should also be improved to overwrite all scratch registers
modified by the function that are not part of the return value or the registers
already preserved by the function.0

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