[Bug 1322287] Re: HTM __builtin_ttest rtl expansion uses wrong shift amount

[Matthias Klose]

** Also affects: gcc-4.8 (Ubuntu Trusty)
   Importance: Undecided
       Status: New

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Title:
  HTM __builtin_ttest rtl expansion uses wrong shift amount

Status in “gcc-4.8” package in Ubuntu:
  New
Status in “gcc-4.8” source package in Trusty:
  New

Bug description:
  There is a semi-latent bug for the HTM ttest pattern used with the
  __builtin_ttest() builtin.  This is supposed to expand to a tabortwci.
  instruction which sets cr0 and then some code that copies the cr0 value
  into a gpr and then shifts and masks it into the lowest 2 bits in the gpr.
  The mfcr and mfocrf instructions which can be used to copy the CR0 value
  into a gpr, both copy the value into bits 32-35 of the gpr.  The bug is
  that we only shift the gpr 24 bits to get the CR value into the low
  order bits of the gpr, when we should be shifting 28 bits.  This "works"
  most of the time due to a peculiarity in how the mfocrf instruction
  works, since it copies the CR value into bits 32-35 and duplicates
  that value in bits 36-39.  Since newish -mcpu targets (eg, power8)
  normally generate a mfocrf, we don't see the problem.  However, in some
  cases, we will instead generate a mfcr instruction, which does expose
  the bug.

  This bug was reported upstream with a patch here:

      https://gcc.gnu.org/ml/gcc-patches/2014-05/msg01667.html

  ...and has been fixed upstream in trunk and the FSF 4.9 and FSF 4.8
  branches, as revisions 210815, 210817 and 210818 respectively.

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