mirror of
https://github.com/c64scene-ar/llvm-6502.git
synced 2024-11-01 00:11:00 +00:00
e47e75b461
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@36594 91177308-0d34-0410-b5e6-96231b3b80d8
145 lines
4.1 KiB
Plaintext
145 lines
4.1 KiB
Plaintext
//===---------------------------------------------------------------------===//
|
|
|
|
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]
|