+//===---------------------------------------------------------------------===//
+
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
+ 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
+ mul lr, r4, lr
mov r4, lr
- str lr, [sp, #+52]
- ldr lr, [r1, #+32]
- sxth r3, r3
- mla r4, r3, lr, r4
+ 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
+ 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.
+
+//===---------------------------------------------------------------------===//
+
+It would be good to detect collector/target compatibility instead of silently
+doing the wrong thing.
+
+//===---------------------------------------------------------------------===//
+
+It would be really nice to be able to write patterns in .td files for copies,
+which would eliminate a bunch of explicit predicates on them (e.g. no side
+effects). Once this is in place, it would be even better to have tblgen
+synthesize the various copy insertion/inspection methods in TargetInstrInfo.
projects / oota-llvm.git / blobdiff