[Qemu-devel] [Bug 870990] Re: compile failure on ARMv7 hosts when compiled for thumb if --enable-debug

[Peter Maydell]

** Also affects: qemu
   Importance: Undecided
       Status: New

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https://bugs.launchpad.net/bugs/870990

Title:
  compile failure on ARMv7 hosts when compiled for thumb if --enable-
  debug

Status in QEMU:
  New
Status in Linaro QEMU:
  Confirmed

Bug description:
  QEMU won't compile if you configure --enable-debug and gcc is building
  in Thumb2.

  This is because we have picked r7 as the TCG_AREG0 (fixed register for
  holding the CPU environment pointer), which clashes with its use as
  the frame pointer in Thumb. (ARM compilation is fine because the frame
  pointer is a different register there.)

  We could fix this by forcing -fomit-frame-pointer when compiling the
  relevant source files (which is what we do on x86 where register
  pressure forces us to use EBP for AREG0) but it would be much better
  to just move AREG0 to something else. We can use r6 if we want to
  stick with a low-reg, or move up to r8 if we think a high-reg likely
  to be a better choice.

  TODO:
   * compile with both r6 and r8, and do a trivial benchmarking run
   * submit patch (before the upstream 1.0 freeze!)

  NB: AREG0 is set in two places that need to stay in sync: dyngen-
  exec.h and tcg/arm/tcg-target.h

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