[Sbcl-devel] Argument X is not a REAL: :INVALID-VALUE-FOR-UNESCAPED-REGISTER-STORAGE

[Juho S. <js...@ik...>]

The debugger has been erroring out on backtraces with the message
"Argument X is not a REAL: :INVALID-VALUE-FOR-UNESCAPED-REGISTER-STORAGE"
for a couple of weeks.  Evaluating (sb-int::sap-ref-word
(sb-int::int-sap 0) 0) and then doing a backtrace seems to be one
(but not the only) reliable method of reproducing this bug, at least
on Linux x86/amd64. This is rather inconvenient, so I'd like to see
at least some sort of fix for the problem before the next release.

The problem seems to be that SB-DEBUG::FRAME-CALL assumes the first
element of (FRAME-ARGS-AS-LIST) contains the argument count for XEPs,
but sometimes contains :INVALID-VALUE-FOR-UNESCAPED-REGISTER-STORAGE
instead. The attached patch checks for this case and ignores the the
count when it's :I-V-F-U-R-S. This fixes the symptoms, but I have no
idea of whether it's the right thing to do.

Does anyone with a better understanding of the debugger have any time
to look at this before the freeze, or should I just commit the
workaround?

-- 
Juho Snellman