llvm-6502/lib/Target/XCore
Quentin Colombet 2f7322b348 [ShrinkWrap] Add (a simplified version) of shrink-wrapping.
This patch introduces a new pass that computes the safe point to insert the
prologue and epilogue of the function.
The interest is to find safe points that are cheaper than the entry and exits
blocks.

As an example and to avoid regressions to be introduce, this patch also
implements the required bits to enable the shrink-wrapping pass for AArch64.


** Context **

Currently we insert the prologue and epilogue of the method/function in the
entry and exits blocks. Although this is correct, we can do a better job when
those are not immediately required and insert them at less frequently executed
places.
The job of the shrink-wrapping pass is to identify such places.


** Motivating example **

Let us consider the following function that perform a call only in one branch of
a if:
define i32 @f(i32 %a, i32 %b)  {
 %tmp = alloca i32, align 4
 %tmp2 = icmp slt i32 %a, %b
 br i1 %tmp2, label %true, label %false

true:
 store i32 %a, i32* %tmp, align 4
 %tmp4 = call i32 @doSomething(i32 0, i32* %tmp)
 br label %false

false:
 %tmp.0 = phi i32 [ %tmp4, %true ], [ %a, %0 ]
 ret i32 %tmp.0
}

On AArch64 this code generates (removing the cfi directives to ease
readabilities):
_f:                                     ; @f
; BB#0:
  stp x29, x30, [sp, #-16]!
  mov  x29, sp
  sub sp, sp, #16             ; =16
  cmp  w0, w1
  b.ge  LBB0_2
; BB#1:                                 ; %true
  stur  w0, [x29, #-4]
  sub x1, x29, #4             ; =4
  mov  w0, wzr
  bl  _doSomething
LBB0_2:                                 ; %false
  mov  sp, x29
  ldp x29, x30, [sp], #16
  ret

With shrink-wrapping we could generate:
_f:                                     ; @f
; BB#0:
  cmp  w0, w1
  b.ge  LBB0_2
; BB#1:                                 ; %true
  stp x29, x30, [sp, #-16]!
  mov  x29, sp
  sub sp, sp, #16             ; =16
  stur  w0, [x29, #-4]
  sub x1, x29, #4             ; =4
  mov  w0, wzr
  bl  _doSomething
  add sp, x29, #16            ; =16
  ldp x29, x30, [sp], #16
LBB0_2:                                 ; %false
  ret

Therefore, we would pay the overhead of setting up/destroying the frame only if
we actually do the call.


** Proposed Solution **

This patch introduces a new machine pass that perform the shrink-wrapping
analysis (See the comments at the beginning of ShrinkWrap.cpp for more details).
It then stores the safe save and restore point into the MachineFrameInfo
attached to the MachineFunction.
This information is then used by the PrologEpilogInserter (PEI) to place the
related code at the right place. This pass runs right before the PEI.

Unlike the original paper of Chow from PLDI’88, this implementation of
shrink-wrapping does not use expensive data-flow analysis and does not need hack
to properly avoid frequently executed point. Instead, it relies on dominance and
loop properties.

The pass is off by default and each target can opt-in by setting the
EnableShrinkWrap boolean to true in their derived class of TargetPassConfig.
This setting can also be overwritten on the command line by using
-enable-shrink-wrap.

Before you try out the pass for your target, make sure you properly fix your
emitProlog/emitEpilog/adjustForXXX method to cope with basic blocks that are not
necessarily the entry block.


** Design Decisions **

1. ShrinkWrap is its own pass right now. It could frankly be merged into PEI but
for debugging and clarity I thought it was best to have its own file.
2. Right now, we only support one save point and one restore point. At some
point we can expand this to several save point and restore point, the impacted
component would then be:
- The pass itself: New algorithm needed.
- MachineFrameInfo: Hold a list or set of Save/Restore point instead of one
  pointer.
- PEI: Should loop over the save point and restore point.
Anyhow, at least for this first iteration, I do not believe this is interesting
to support the complex cases. We should revisit that when we motivating
examples.

Differential Revision: http://reviews.llvm.org/D9210

<rdar://problem/3201744>


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@236507 91177308-0d34-0410-b5e6-96231b3b80d8
2015-05-05 17:38:16 +00:00
..
Disassembler Pass an ArrayRef to MCDisassembler::getInstruction. 2014-11-12 02:04:27 +00:00
InstPrinter [MCInstPrinter] Enable MCInstPrinter to change its behavior based on the 2015-03-27 20:36:02 +00:00
MCTargetDesc Replace the MCSubtargetInfo parameter with a Triple when creating 2015-03-31 00:10:04 +00:00
TargetInfo
CMakeLists.txt [PM] Switch the TargetMachine interface from accepting a pass manager 2015-01-31 11:17:59 +00:00
LLVMBuild.txt
Makefile
README.txt
XCore.h [PM] Remove a bunch of stale TTI creation method declarations. I nuked 2015-02-01 00:22:15 +00:00
XCore.td
XCoreAsmPrinter.cpp [AsmPrinter] Make AsmPrinter's OutStreamer member a unique_ptr. 2015-04-24 19:11:51 +00:00
XCoreCallingConv.td
XCoreFrameLowering.cpp [ShrinkWrap] Add (a simplified version) of shrink-wrapping. 2015-05-05 17:38:16 +00:00
XCoreFrameLowering.h [ShrinkWrap] Add (a simplified version) of shrink-wrapping. 2015-05-05 17:38:16 +00:00
XCoreFrameToArgsOffsetElim.cpp Have MachineFunction cache a pointer to the subtarget to make lookups 2014-08-05 02:39:49 +00:00
XCoreInstrFormats.td
XCoreInstrInfo.cpp Work around bugs in MSVC "14" CTP 3's conversion logic 2014-10-31 23:19:46 +00:00
XCoreInstrInfo.h Canonicalize header guards into a common format. 2014-08-13 16:26:38 +00:00
XCoreInstrInfo.td Reapply r235977 "[DebugInfo] Add debug locations to constant SD nodes" 2015-04-28 14:05:47 +00:00
XCoreISelDAGToDAG.cpp Reapply r235977 "[DebugInfo] Add debug locations to constant SD nodes" 2015-04-28 14:05:47 +00:00
XCoreISelLowering.cpp Reapply r235977 "[DebugInfo] Add debug locations to constant SD nodes" 2015-04-28 14:05:47 +00:00
XCoreISelLowering.h Make each target map all inline assembly memory constraints to InlineAsm::Constraint_m. NFC. 2015-03-16 13:13:41 +00:00
XCoreLowerThreadLocal.cpp [opaque pointer type] More GEP IRBuilder API migrations... 2015-04-03 21:33:42 +00:00
XCoreMachineFunctionInfo.cpp
XCoreMachineFunctionInfo.h Canonicalize header guards into a common format. 2014-08-13 16:26:38 +00:00
XCoreMCInstLower.cpp
XCoreMCInstLower.h Canonicalize header guards into a common format. 2014-08-13 16:26:38 +00:00
XCoreRegisterInfo.cpp Have getCalleeSavedRegs take a non-null MachineFunction all the 2015-03-11 21:41:28 +00:00
XCoreRegisterInfo.h Have getCalleeSavedRegs take a non-null MachineFunction all the 2015-03-11 21:41:28 +00:00
XCoreRegisterInfo.td
XCoreSelectionDAGInfo.cpp Have MachineFunction cache a pointer to the subtarget to make lookups 2014-08-05 02:39:49 +00:00
XCoreSelectionDAGInfo.h Canonicalize header guards into a common format. 2014-08-13 16:26:38 +00:00
XCoreSubtarget.cpp Since TargetLowering is already subtarget dependent just pass 2015-02-02 17:52:27 +00:00
XCoreSubtarget.h Move DataLayout back to the TargetMachine from TargetSubtargetInfo 2015-01-26 19:03:15 +00:00
XCoreTargetMachine.cpp Move the DataLayout to the generic TargetMachine, making it mandatory. 2015-03-12 00:07:24 +00:00
XCoreTargetMachine.h Remove the target independent TargetMachine::getSubtarget and 2015-03-21 04:22:23 +00:00
XCoreTargetObjectFile.cpp Compute the ELF SectionKind from the flags. 2015-01-29 17:33:21 +00:00
XCoreTargetObjectFile.h Canonicalize header guards into a common format. 2014-08-13 16:26:38 +00:00
XCoreTargetStreamer.h Use 'override/final' instead of 'virtual' for overridden methods 2015-04-11 02:11:45 +00:00
XCoreTargetTransformInfo.h [multiversion] Switch the TTI queries from TargetMachine to Subtarget 2015-02-01 14:22:17 +00:00

To-do
-----

* Instruction encodings
* Tailcalls
* Investigate loop alignment
* Add builtins