On November 16, 2017 7:05:30 PM GMT+01:00, Jeff Law <l...@redhat.com> wrote: >On 11/16/2017 05:34 AM, Alan Hayward wrote: >> This is a set of patches aimed at supporting aarch64 SVE register >> preservation around TLS calls. >> >> Across a TLS call, Aarch64 SVE does not explicitly preserve the >> SVE vector registers. However, the Neon vector registers are >preserved. >> Due to overlapping of registers, this means the lower 128bits of all >> SVE vector registers will be preserved. >> >> The existing GCC code will currently incorrectly assume preservation >> of all of the SVE registers. >> >> This patch introduces a CLOBBER_HIGH expression. This behaves a bit >like >> a CLOBBER expression. CLOBBER_HIGH can only refer to a single >register. >> The mode of the expression indicates the size of the lower bits which >> will be preserved. If the register contains a value bigger than this >> mode then the code will treat the register as clobbered. >> >> The means in order to evaluate if a clobber high is relevant, we need >to ensure >> the mode of the existing value in a register is tracked. >> >> The following patches in this series add support for the >CLOBBER_HIGH, >> with the final patch adding CLOBBER_HIGHs around TLS_DESC calls for >> aarch64. The testing performed on these patches is also detailed in >the >> final patch. >> >> These patches are based on top of the linaro-dev/sve branch. >> >> A simpler alternative to this patch would be to assume all Neon and >SVE >> registers are clobbered across TLS calls, however this would be a >> performance regression against all Aarch64 targets. >So just a couple design questions. > >Presumably there's no reasonable way to set up GCC's view of the >register file to avoid this problem? ISTM that if the SVE register was >split into two, one for the part that overlapped with the neon register >and one that did not, then this could be handled via standard >mechanisms? > >Alternately would it be easier to clobber a subreg representing the >high >part of the register? Hmm, probably not.
I thought of a set of the preserved part to itself that leaves the upper part undefined. Not sure if we have such thing or if it would work in all places that a clobber does. Richard. >Jeff
