[Bug translator/27829] New: support for floating point values passed through sdt.h markers

["fche at redhat dot com" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=27829

            Bug ID: 27829
           Summary: support for floating point values passed through sdt.h
                    markers
           Product: systemtap
           Version: unspecified
            Status: NEW
          Severity: normal
          Priority: P2
         Component: translator
          Assignee: systemtap at sourceware dot org
          Reporter: fche at redhat dot com
  Target Milestone: ---

Created attachment 13422
  --> https://sourceware.org/bugzilla/attachment.cgi?id=13422&action=edit
partial patch

While stap has recently developed the ability access floating point parameters
passed through dwarf-designated locations, we can't do that for sdt.h.  That's
for a couple of reasons: 

- tapsets.cxx sdt_uprobe_var_expanding_visitor::build_dwarf_registers does not
recognize FP register names
- includes/sys/sdt.h uses an STAP_SDT_ARG_CONSTRAINT that excludes FP registers
(so FP values are copied into integer registers anyway)
- includes/sys/sdt.h has no way of marking up floating-point vs integer
registers

A partial patch for the first two parts is attached, for x86-64.
The latter requires some coordination with other sdt.h consumers, in order to
parse a modification of the operand description syntax.  One possibility:

Instead of:

% readelf -n stap
stapsdt              0x0000003b       NT_STAPSDT (SystemTap probe descriptors)
[...]
    Arguments: 8@%xmm0 4@%xmm1

use:

stapsdt              0x0000003b       NT_STAPSDT (SystemTap probe descriptors)
[...]
    Arguments: 8f@%xmm0 4f@%xmm1

i.e., insert an "f" char between the length and the "@" sign.  We could/should
do that even if the floats were copied into an integer register due to operand
constraints.

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