llvm-6502/lib/CodeGen
Owen Anderson afc6de0015 A little more progress on StrongPHIElimination, now that I have a better sense of
how the CodeGen machinery works.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@44786 91177308-0d34-0410-b5e6-96231b3b80d8
2007-12-10 08:07:09 +00:00
..
SelectionDAG Duncan points out that the subtraction is unneeded since hte code 2007-12-09 17:56:34 +00:00
AsmPrinter.cpp Fix PIC jump table codegen on x86-32/linux. In fact, such thing should be applied 2007-11-14 09:18:41 +00:00
BranchFolding.cpp Improve branch folding by recgonizing that explict successor relationships impact the value of fall-through choices. 2007-12-10 07:24:06 +00:00
Collector.cpp Collector is the base class for garbage collection code generators. 2007-09-29 02:13:43 +00:00
CollectorMetadata.cpp CollectorMetadata abstractly describes stack maps for a function. 2007-09-27 22:18:46 +00:00
Collectors.cpp My previous Registry.h header, as well as Collectors.h, which is the 2007-09-27 19:34:27 +00:00
DwarfWriter.cpp Fix .eh table linkage issues on Darwin. Some EH support 2007-11-20 23:24:42 +00:00
ELFWriter.cpp Eliminate the remaining uses of getTypeSize. This 2007-11-05 00:04:43 +00:00
ELFWriter.h Here is the bulk of the sanitizing. 2007-07-05 17:07:56 +00:00
IfConversion.cpp Somehow this wasn't committed last time. M_CLOBBERS_PRED is gone. 2007-07-10 17:50:43 +00:00
IntrinsicLowering.cpp Implement necessary bits for flt_rounds gcc builtin. 2007-11-15 23:25:33 +00:00
LiveInterval.cpp Replace the odd kill# hack with something less fragile. 2007-11-29 09:49:23 +00:00
LiveIntervalAnalysis.cpp Reverting 44702. It wasn't correct to rename them. 2007-12-08 23:58:46 +00:00
LiveVariables.cpp Live interval splitting: 2007-11-17 00:40:40 +00:00
LLVMTargetMachine.cpp Initial commit of the machine code LICM pass. It successfully hoists this: 2007-12-07 21:42:31 +00:00
LowerSubregs.cpp isSubRegOf() is a dup of isSubRegister. 2007-10-23 06:51:50 +00:00
MachineBasicBlock.cpp Use empty() member functions when that's what's being tested for instead 2007-10-03 19:26:29 +00:00
MachineDominators.cpp Add a newline at the end of the file. 2007-10-31 08:49:24 +00:00
MachineFunction.cpp Eliminate the remaining uses of getTypeSize. This 2007-11-05 00:04:43 +00:00
MachineInstr.cpp Clean up sub-register implementation by moving subReg information back to 2007-11-14 07:59:08 +00:00
MachineLICM.cpp Reverting 44702. It wasn't correct to rename them. 2007-12-08 23:58:46 +00:00
MachineLoopInfo.cpp Add missing newlines at EOF. 2007-11-28 10:13:38 +00:00
MachineModuleInfo.cpp Provide a way to update DescGlobals cache directly. 2007-11-30 00:51:33 +00:00
MachinePassRegistry.cpp
MachOWriter.cpp Eliminate the remaining uses of getTypeSize. This 2007-11-05 00:04:43 +00:00
MachOWriter.h Eliminate the remaining uses of getTypeSize. This 2007-11-05 00:04:43 +00:00
Makefile this will work better 2006-11-03 19:15:55 +00:00
Passes.cpp *** empty log message *** 2006-11-16 20:11:33 +00:00
PHIElimination.cpp Bring UsedBlocks back. StrongPHIElimination needs this information. 2007-11-08 01:20:48 +00:00
PhysRegTracker.h Add explicit keywords and remove spurious trailing semicolons. 2007-08-27 14:50:10 +00:00
PostRASchedulerList.cpp Modify previous patch per review comments. 2007-07-13 17:31:29 +00:00
PrologEpilogInserter.cpp Add a argument to storeRegToStackSlot and storeRegToAddr to specify whether 2007-12-05 03:14:33 +00:00
README.txt Changed XXX to FIXME, and added comment to the README file 2007-10-25 19:49:32 +00:00
RegAllocBigBlock.cpp Add a argument to storeRegToStackSlot and storeRegToAddr to specify whether 2007-12-05 03:14:33 +00:00
RegAllocLinearScan.cpp Live interval splitting: 2007-11-17 00:40:40 +00:00
RegAllocLocal.cpp Add a argument to storeRegToStackSlot and storeRegToAddr to specify whether 2007-12-05 03:14:33 +00:00
RegAllocSimple.cpp Add a argument to storeRegToStackSlot and storeRegToAddr to specify whether 2007-12-05 03:14:33 +00:00
RegisterCoalescer.cpp Pluggable coalescers inplementation. 2007-09-06 16:18:45 +00:00
RegisterScavenging.cpp Add a argument to storeRegToStackSlot and storeRegToAddr to specify whether 2007-12-05 03:14:33 +00:00
SimpleRegisterCoalescing.cpp Add an option to control this heuristic tweak so I can test it. 2007-12-07 00:28:32 +00:00
SimpleRegisterCoalescing.h First step towards moving the coalescer to priority_queue based machinery. 2007-11-06 08:52:21 +00:00
StrongPHIElimination.cpp A little more progress on StrongPHIElimination, now that I have a better sense of 2007-12-10 08:07:09 +00:00
TwoAddressInstructionPass.cpp As Chris and Evan pointed out, BreakCriticalMachineEdges doesn't really need 2007-11-12 01:05:09 +00:00
UnreachableBlockElim.cpp Fix typo in comment. 2007-05-06 13:37:16 +00:00
VirtRegMap.cpp MachineInstr can change. Store indexes instead. 2007-12-05 10:24:35 +00:00
VirtRegMap.h MachineInstr can change. Store indexes instead. 2007-12-05 10:24:35 +00:00

//===---------------------------------------------------------------------===//

Common register allocation / spilling problem:

        mul lr, r4, lr
        str lr, [sp, #+52]
        ldr lr, [r1, #+32]
        sxth r3, r3
        ldr r4, [sp, #+52]
        mla r4, r3, lr, r4

can be:

        mul lr, r4, lr
        mov r4, lr
        str lr, [sp, #+52]
        ldr lr, [r1, #+32]
        sxth r3, r3
        mla r4, r3, lr, r4

and then "merge" mul and mov:

        mul r4, r4, lr
        str lr, [sp, #+52]
        ldr lr, [r1, #+32]
        sxth r3, r3
        mla r4, r3, lr, r4

It also increase the likelyhood the store may become dead.

//===---------------------------------------------------------------------===//

I think we should have a "hasSideEffects" flag (which is automatically set for
stuff that "isLoad" "isCall" etc), and the remat pass should eventually be able
to remat any instruction that has no side effects, if it can handle it and if
profitable.

For now, I'd suggest having the remat stuff work like this:

1. I need to spill/reload this thing.
2. Check to see if it has side effects.
3. Check to see if it is simple enough: e.g. it only has one register
destination and no register input.
4. If so, clone the instruction, do the xform, etc.

Advantages of this are:

1. the .td file describes the behavior of the instructions, not the way the
   algorithm should work.
2. as remat gets smarter in the future, we shouldn't have to be changing the .td
   files.
3. it is easier to explain what the flag means in the .td file, because you
   don't have to pull in the explanation of how the current remat algo works.

Some potential added complexities:

1. Some instructions have to be glued to it's predecessor or successor. All of
   the PC relative instructions and condition code setting instruction. We could
   mark them as hasSideEffects, but that's not quite right. PC relative loads
   from constantpools can be remat'ed, for example. But it requires more than
   just cloning the instruction. Some instructions can be remat'ed but it
   expands to more than one instruction. But allocator will have to make a
   decision.

4. As stated in 3, not as simple as cloning in some cases. The target will have
   to decide how to remat it. For example, an ARM 2-piece constant generation
   instruction is remat'ed as a load from constantpool.

//===---------------------------------------------------------------------===//

bb27 ...
        ...
        %reg1037 = ADDri %reg1039, 1
        %reg1038 = ADDrs %reg1032, %reg1039, %NOREG, 10
    Successors according to CFG: 0x8b03bf0 (#5)

bb76 (0x8b03bf0, LLVM BB @0x8b032d0, ID#5):
    Predecessors according to CFG: 0x8b0c5f0 (#3) 0x8b0a7c0 (#4)
        %reg1039 = PHI %reg1070, mbb<bb76.outer,0x8b0c5f0>, %reg1037, mbb<bb27,0x8b0a7c0>

Note ADDri is not a two-address instruction. However, its result %reg1037 is an
operand of the PHI node in bb76 and its operand %reg1039 is the result of the
PHI node. We should treat it as a two-address code and make sure the ADDri is
scheduled after any node that reads %reg1039.

//===---------------------------------------------------------------------===//

Use local info (i.e. register scavenger) to assign it a free register to allow
reuse:
	ldr r3, [sp, #+4]
	add r3, r3, #3
	ldr r2, [sp, #+8]
	add r2, r2, #2
	ldr r1, [sp, #+4]  <==
	add r1, r1, #1
	ldr r0, [sp, #+4]
	add r0, r0, #2

//===---------------------------------------------------------------------===//

LLVM aggressively lift CSE out of loop. Sometimes this can be negative side-
effects:

R1 = X + 4
R2 = X + 7
R3 = X + 15

loop:
load [i + R1]
...
load [i + R2]
...
load [i + R3]

Suppose there is high register pressure, R1, R2, R3, can be spilled. We need
to implement proper re-materialization to handle this:

R1 = X + 4
R2 = X + 7
R3 = X + 15

loop:
R1 = X + 4  @ re-materialized
load [i + R1]
...
R2 = X + 7 @ re-materialized
load [i + R2]
...
R3 = X + 15 @ re-materialized
load [i + R3]

Furthermore, with re-association, we can enable sharing:

R1 = X + 4
R2 = X + 7
R3 = X + 15

loop:
T = i + X
load [T + 4]
...
load [T + 7]
...
load [T + 15]
//===---------------------------------------------------------------------===//

It's not always a good idea to choose rematerialization over spilling. If all
the load / store instructions would be folded then spilling is cheaper because
it won't require new live intervals / registers. See 2003-05-31-LongShifts for
an example.

//===---------------------------------------------------------------------===//

With a copying garbage collector, derived pointers must not be retained across
collector safe points; the collector could move the objects and invalidate the
derived pointer. This is bad enough in the first place, but safe points can
crop up unpredictably. Consider:

        %array = load { i32, [0 x %obj] }** %array_addr
        %nth_el = getelementptr { i32, [0 x %obj] }* %array, i32 0, i32 %n
        %old = load %obj** %nth_el
        %z = div i64 %x, %y
        store %obj* %new, %obj** %nth_el

If the i64 division is lowered to a libcall, then a safe point will (must)
appear for the call site. If a collection occurs, %array and %nth_el no longer
point into the correct object.

The fix for this is to copy address calculations so that dependent pointers
are never live across safe point boundaries. But the loads cannot be copied
like this if there was an intervening store, so may be hard to get right.

Only a concurrent mutator can trigger a collection at the libcall safe point.
So single-threaded programs do not have this requirement, even with a copying
collector. Still, LLVM optimizations would probably undo a front-end's careful
work.

//===---------------------------------------------------------------------===//

The ocaml frametable structure supports liveness information. It would be good
to support it.

//===---------------------------------------------------------------------===//

The FIXME in ComputeCommonTailLength in BranchFolding.cpp needs to be
revisited. The check is there to work around a misuse of directives in inline
assembly.

//===---------------------------------------------------------------------===//