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200 lines
6.5 KiB
Plaintext
200 lines
6.5 KiB
Plaintext
//===---------------------------------------------------------------------===//
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Common register allocation / spilling problem:
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mul lr, r4, lr
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str lr, [sp, #+52]
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ldr lr, [r1, #+32]
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sxth r3, r3
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ldr r4, [sp, #+52]
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mla r4, r3, lr, r4
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can be:
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mul lr, r4, lr
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mov r4, lr
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str lr, [sp, #+52]
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ldr lr, [r1, #+32]
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sxth r3, r3
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mla r4, r3, lr, r4
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and then "merge" mul and mov:
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mul r4, r4, lr
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str lr, [sp, #+52]
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ldr lr, [r1, #+32]
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sxth r3, r3
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mla r4, r3, lr, r4
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It also increase the likelyhood the store may become dead.
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//===---------------------------------------------------------------------===//
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I think we should have a "hasSideEffects" flag (which is automatically set for
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stuff that "isLoad" "isCall" etc), and the remat pass should eventually be able
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to remat any instruction that has no side effects, if it can handle it and if
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profitable.
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For now, I'd suggest having the remat stuff work like this:
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1. I need to spill/reload this thing.
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2. Check to see if it has side effects.
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3. Check to see if it is simple enough: e.g. it only has one register
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destination and no register input.
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4. If so, clone the instruction, do the xform, etc.
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Advantages of this are:
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1. the .td file describes the behavior of the instructions, not the way the
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algorithm should work.
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2. as remat gets smarter in the future, we shouldn't have to be changing the .td
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files.
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3. it is easier to explain what the flag means in the .td file, because you
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don't have to pull in the explanation of how the current remat algo works.
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Some potential added complexities:
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1. Some instructions have to be glued to it's predecessor or successor. All of
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the PC relative instructions and condition code setting instruction. We could
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mark them as hasSideEffects, but that's not quite right. PC relative loads
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from constantpools can be remat'ed, for example. But it requires more than
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just cloning the instruction. Some instructions can be remat'ed but it
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expands to more than one instruction. But allocator will have to make a
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decision.
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4. As stated in 3, not as simple as cloning in some cases. The target will have
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to decide how to remat it. For example, an ARM 2-piece constant generation
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instruction is remat'ed as a load from constantpool.
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//===---------------------------------------------------------------------===//
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bb27 ...
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...
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%reg1037 = ADDri %reg1039, 1
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%reg1038 = ADDrs %reg1032, %reg1039, %NOREG, 10
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Successors according to CFG: 0x8b03bf0 (#5)
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bb76 (0x8b03bf0, LLVM BB @0x8b032d0, ID#5):
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Predecessors according to CFG: 0x8b0c5f0 (#3) 0x8b0a7c0 (#4)
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%reg1039 = PHI %reg1070, mbb<bb76.outer,0x8b0c5f0>, %reg1037, mbb<bb27,0x8b0a7c0>
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Note ADDri is not a two-address instruction. However, its result %reg1037 is an
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operand of the PHI node in bb76 and its operand %reg1039 is the result of the
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PHI node. We should treat it as a two-address code and make sure the ADDri is
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scheduled after any node that reads %reg1039.
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//===---------------------------------------------------------------------===//
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Use local info (i.e. register scavenger) to assign it a free register to allow
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reuse:
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ldr r3, [sp, #+4]
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add r3, r3, #3
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ldr r2, [sp, #+8]
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add r2, r2, #2
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ldr r1, [sp, #+4] <==
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add r1, r1, #1
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ldr r0, [sp, #+4]
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add r0, r0, #2
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//===---------------------------------------------------------------------===//
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LLVM aggressively lift CSE out of loop. Sometimes this can be negative side-
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effects:
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R1 = X + 4
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R2 = X + 7
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R3 = X + 15
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loop:
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load [i + R1]
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...
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load [i + R2]
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...
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load [i + R3]
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Suppose there is high register pressure, R1, R2, R3, can be spilled. We need
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to implement proper re-materialization to handle this:
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R1 = X + 4
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R2 = X + 7
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R3 = X + 15
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loop:
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R1 = X + 4 @ re-materialized
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load [i + R1]
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...
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R2 = X + 7 @ re-materialized
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load [i + R2]
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...
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R3 = X + 15 @ re-materialized
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load [i + R3]
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Furthermore, with re-association, we can enable sharing:
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R1 = X + 4
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R2 = X + 7
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R3 = X + 15
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loop:
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T = i + X
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load [T + 4]
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...
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load [T + 7]
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...
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load [T + 15]
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//===---------------------------------------------------------------------===//
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It's not always a good idea to choose rematerialization over spilling. If all
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the load / store instructions would be folded then spilling is cheaper because
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it won't require new live intervals / registers. See 2003-05-31-LongShifts for
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an example.
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//===---------------------------------------------------------------------===//
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With a copying garbage collector, derived pointers must not be retained across
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collector safe points; the collector could move the objects and invalidate the
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derived pointer. This is bad enough in the first place, but safe points can
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crop up unpredictably. Consider:
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%array = load { i32, [0 x %obj] }** %array_addr
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%nth_el = getelementptr { i32, [0 x %obj] }* %array, i32 0, i32 %n
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%old = load %obj** %nth_el
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%z = div i64 %x, %y
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store %obj* %new, %obj** %nth_el
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If the i64 division is lowered to a libcall, then a safe point will (must)
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appear for the call site. If a collection occurs, %array and %nth_el no longer
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point into the correct object.
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The fix for this is to copy address calculations so that dependent pointers
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are never live across safe point boundaries. But the loads cannot be copied
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like this if there was an intervening store, so may be hard to get right.
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Only a concurrent mutator can trigger a collection at the libcall safe point.
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So single-threaded programs do not have this requirement, even with a copying
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collector. Still, LLVM optimizations would probably undo a front-end's careful
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work.
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//===---------------------------------------------------------------------===//
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The ocaml frametable structure supports liveness information. It would be good
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to support it.
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//===---------------------------------------------------------------------===//
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The FIXME in ComputeCommonTailLength in BranchFolding.cpp needs to be
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revisited. The check is there to work around a misuse of directives in inline
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assembly.
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//===---------------------------------------------------------------------===//
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It would be good to detect collector/target compatibility instead of silently
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doing the wrong thing.
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//===---------------------------------------------------------------------===//
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It would be really nice to be able to write patterns in .td files for copies,
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which would eliminate a bunch of explicit predicates on them (e.g. no side
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effects). Once this is in place, it would be even better to have tblgen
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synthesize the various copy insertion/inspection methods in TargetInstrInfo.
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