This patch changes the location of the frame-record (FP, LR) to the
bottom of the callee-saved area. According to the AAPCS the location of
the frame-record within the stackframe is unspecified (section 5.2.3 The
Frame Pointer), so the compiler should be free to choose a different
The reason for changing the location of the frame-record is to prepare
the frame for allocating an SVE area below the callee-saves. This way the
compiler can use the VL-scaled addressing modes to directly access SVE
objects from the frame-pointer.
: : | stack | | stack | | args | | args | +-------+ +-------+ | x30 | | x19 | | x29 | | x20 | FP -> |- - - -| | x21 | | x19 | ==> | x22 | | x20 | |- - - -| | x21 | | x30 | | x22 | | x29 | +-------+ +-------+ <- FP |///////| |///////| // realignment gap |- - - -| |- - - -| |spills/| |spills/| | locals| | locals| SP -> +-------+ +-------+ <- SP
Things to point out:
- The CFI instructions emit the FP/LR registers first (in the order as specified in the CallingConvention.td file) in order to support the Compact Unwinding format.
- The algorith to find a paired register should be prevented from accidentally pairing some callee-saved register with LR that is not FP, since they should always be paired together when the frame has a frame-record.