200 lines
6.2 KiB
Plaintext
200 lines
6.2 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 r4, [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 likelihood the store may become dead.
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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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//===---------------------------------------------------------------------===//
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Stack coloring improvements:
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1. Do proper LiveStacks analysis on all stack objects including those which are
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not spill slots.
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2. Reorder objects to fill in gaps between objects.
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e.g. 4, 1, <gap>, 4, 1, 1, 1, <gap>, 4 => 4, 1, 1, 1, 1, 4, 4
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//===---------------------------------------------------------------------===//
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The scheduler should be able to sort nearby instructions by their address. For
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example, in an expanded memset sequence it's not uncommon to see code like this:
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movl $0, 4(%rdi)
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movl $0, 8(%rdi)
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movl $0, 12(%rdi)
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movl $0, 0(%rdi)
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Each of the stores is independent, and the scheduler is currently making an
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arbitrary decision about the order.
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//===---------------------------------------------------------------------===//
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Another opportunitiy in this code is that the $0 could be moved to a register:
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movl $0, 4(%rdi)
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movl $0, 8(%rdi)
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movl $0, 12(%rdi)
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movl $0, 0(%rdi)
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This would save substantial code size, especially for longer sequences like
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this. It would be easy to have a rule telling isel to avoid matching MOV32mi
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if the immediate has more than some fixed number of uses. It's more involved
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to teach the register allocator how to do late folding to recover from
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excessive register pressure.
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