f7d8336a2f
This patch is in preparation to enable setting the MachineInstr::MIFlag
flags, i.e. FrameSetup/FrameDestroy, on callee saved register
spill/reload instructions in prologue/epilogue. This eventually helps in
setting the prologue_end and epilogue_begin markers more accurately.
The DWARF Spec in "6.4 Call Frame Information" says:
The code that allocates space on the call frame stack and performs the
save
operation is called the subroutine’s prologue, and the code that
performs
the restore operation and deallocates the frame is called its epilogue.
which means the callee saved register spills and reloads are part of
prologue (a.k.a frame setup) and epilogue (a.k.a frame destruction),
respectively. And, IIUC, LLVM backend uses FrameSetup/FrameDestroy flags
to identify instructions that are part of call frame setup and
destruction.
In the trunk, while most targets consistently set
FrameSetup/FrameDestroy on save/restore call frame information (CFI)
instructions of callee saved registers, they do not consistently set
those flags on the actual callee saved register spill/reload
instructions.
I believe this patch provides a clean mechanism to set
FrameSetup/FrameDestroy flags on the actual callee saved register
spill/reload instructions as needed. And, by having default argument of
MachineInstr::NoFlags for Flags, this patch is a NFC.
With this patch, the targets have to just pass FrameSetup/FrameDestroy
flag to the storeRegToStackSlot/loadRegFromStackSlot calls from the
target derived spillCalleeSavedRegisters and restoreCalleeSavedRegisters
to set those flags on callee saved register spill/reload instructions.
Also, this patch makes it very easy to set the source line information
on callee saved register spill/reload instructions which is needed by
the DwarfDebug.cpp implementation to set prologue_end and epilogue_begin
markers more accurately.
As per DwarfDebug.cpp implementation:
prologue_end is the first known non-DBG_VALUE and non-FrameSetup
location
that marks the beginning of the function body
epilogue_begin is the first FrameDestroy location that has been seen in
the
epilogue basic block
With this patch, the targets have to just do the following to set the
source line information on callee saved register spill/reload
instructions, without hampering the LLVM's efforts to avoid adding
source line information on the artificial code generated by the
compiler.
<Foo>InstrInfo::storeRegToStackSlot() {
...
DebugLoc DL =
Flags & MachineInstr::FrameSetup ? DebugLoc() : MBB.findDebugLoc(I);
...
}
<Foo>InstrInfo::loadRegFromStackSlot() {
...
DebugLoc DL =
Flags & MachineInstr::FrameDestroy ? MBB.findDebugLoc(I) : DebugLoc();
...
}
While I understand this patch would break out-of-tree backend builds, I
think it is in the right direction.
One immediate use case that can benefit from this patch is fixing
#120553 becomes simpler.
//===---------------------------------------------------------------------===// // MSP430 backend. //===---------------------------------------------------------------------===// DISCLAIMER: This backend should be considered as highly experimental. I never seen nor worked with this MCU, all information was gathered from datasheet only. The original intention of making this backend was to write documentation of form "How to write backend for dummies" :) Thes notes hopefully will be available pretty soon. Some things are incomplete / not implemented yet (this list surely is not complete as well): 1. Verify, how stuff is handling implicit zext with 8 bit operands (this might be modelled currently in improper way - should we need to mark the superreg as def for every 8 bit instruction?). 2. Libcalls: multiplication, division, remainder. Note, that calling convention for libcalls is incomptible with calling convention of libcalls of msp430-gcc (these cannot be used though due to license restriction). 3. Implement multiplication / division by constant (dag combiner hook?). 4. Implement non-constant shifts. 5. Implement varargs stuff. 6. Verify and fix (if needed) how's stuff playing with i32 / i64. 7. Implement floating point stuff (softfp?) 8. Implement instruction encoding for (possible) direct code emission in the future. 9. Since almost all instructions set flags - implement brcond / select in better way (currently they emit explicit comparison). 10. Handle imm in comparisons in better way (see comment in MSP430InstrInfo.td) 11. Implement hooks for better memory op folding, etc.