In particular, don't spill dirty registers only to satisfy a hint. It is
not worth it.
The attached test case provides an example where the fast allocator
would spill a register when other registers are available.
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When compiling a program with lots of small functions like
483.xalancbmk, this makes RAFast 11% faster.
Add some comments to clarify the difference between unallocatable and
reserved registers. It's quite subtle.
The fast register allocator depends on EFLAGS' not being allocatable on
x86. That way it can completely avoid tracking liveness, and it won't
mind when there are multiple uses of a single def.
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registers for fast allocation a different way. This has us updating
used registers only when we're using that exact register.
Fixes rdar://9207598
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These functions not longer assert when passed 0, but simply return false instead.
No functional change intended.
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when no virtual registers have been allocated.
It was only used to resize IndexedMaps, so provide an IndexedMap::resize()
method such that
Map.grow(MRI.getLastVirtReg());
can be replaced with the simpler
Map.resize(MRI.getNumVirtRegs());
This works correctly when no virtuals are allocated, and it bypasses the to/from
index conversions.
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Print virtual registers numbered from 0 instead of the arbitrary
FirstVirtualRegister. The first virtual register is printed as %vreg0.
TRI::NoRegister is printed as %noreg.
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must be called in the pass's constructor. This function uses static dependency declarations to recursively initialize
the pass's dependencies.
Clients that only create passes through the createFooPass() APIs will require no changes. Clients that want to use the
CommandLine options for passes will need to manually call the appropriate initialization functions in PassInitialization.h
before parsing commandline arguments.
I have tested this with all standard configurations of clang and llvm-gcc on Darwin. It is possible that there are problems
with the static dependencies that will only be visible with non-standard options. If you encounter any crash in pass
registration/creation, please send the testcase to me directly.
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overload UserInInstr. Explicitly check Allocatable. The early exit in the
condition will mean the performance impact of the extra test should be
minimal.
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multiple defs, like t2LDRSB_POST.
The first def could accidentally steal the physreg that the second, tied def was
required to be allocated to.
Now, the tied use-def is treated more like an early clobber, and the physreg is
reserved before allocating the other defs.
This would never be a problem when the tied def was the only def which is the
usual case.
This fixes MallocBench/gs for thumb2 -O0.
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This code is transitional, it will soon be possible to eliminate
isExtractSubreg, isInsertSubreg, and isMoveInstr in most places.
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A partial redefine needs to be treated like a tied operand, and the register
must be reloaded while processing use operands.
This fixes a bug where partially redefined registers were processed as normal
defs with a reload added. The reload could clobber another use operand if it was
a kill that allowed register reuse.
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When an instruction has tied operands and physreg defines, we must take extra
care that the tied operands conflict with neither physreg defs nor uses.
The special treatment is given to inline asm and instructions with tied operands
/ early clobbers and physreg defines.
This fixes PR7509.
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Early clobbers defining a virtual register were first alocated to a physreg and
then processed as a physreg EC, spilling the virtreg.
This fixes PR7382.
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register allocation.
Process all of the clobber lists at the end of the function, marking the
registers as used in MachineRegisterInfo.
This is necessary in case the calls clobber callee-saved registers (sic).
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A partial redef now triggers a reload if required. Also don't add
<imp-def,dead> operands for physical superregisters.
Kill flags are still treated as full register kills, and <imp-use,kill> operands
are added for physical superregisters as before.
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This fixes the miscompilations of MultiSource/Applications/JM/l{en,de}cod.
Clang now successfully self hosts in a debug build with the fast register allocator.
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While that approach works wonders for register pressure, it tends to break
everything.
This should unbreak the arm-linux builder and fix a number of miscompilations.
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out aliases when allocating. Clean up allocVirtReg().
Use calcSpillCost() to allow more aggressive hinting. Now the hint is always
taken unless blocked by a reserved register. This leads to more coalescing,
lower register pressure, and less spilling.
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This is safe to do because the physreg has been marked UsedInInstr and the kill flag will be set on the last operand using the virtreg if there are more then one.
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Debug code doesn't use callee saved registers anyway, and the code is simpler this way. Now spillVirtReg always kills, and the isKill parameter is not needed.
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a condition's grouping. Every other use of Allocatable.test(Hint) groups it the
same way as it is indented, so move the parentheses to agree with that
grouping.
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When working top-down in a basic block, substituting physregs for virtregs, the use-def chains are kept up to date. That means we can recognize a virtreg kill by the use-def chain becoming empty.
This makes the fast allocator independent of incoming kill flags.
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- Kill is implicit when use and def registers are identical.
- Only virtual registers can differ.
Add a -verify-fast-regalloc to run the verifier before the fast allocator.
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This loop is quadratic in the capacity for a DenseMap:
while(!map.empty())
map.erase(map.begin());
Instead we now do a normal begin() - end() iteration followed by map.clear().
That also has the nice sideeffect of shrinking the map capacity on demand.
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This allows us to add accurate kill markers, something the scavenger likes.
Add some more tests from ARM that needed this.
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Sorry for the big change. The path leading up to this patch had some TableGen
changes that I didn't want to commit before I knew they were useful. They
weren't, and this version does not need them.
The fast register allocator now does no liveness calculations. Instead it relies
on kill flags provided by isel. (Currently those kill flags are also ignored due
to isel bugs). The allocation algorithm is supposed to work with any subset of
valid kill flags. More kill flags simply means fewer spills inserted.
Registers are allocated from a working set that contains no aliases. That means
most allocations can be done directly without expensive alias checks. When the
working set runs out of registers we do the full alias check to find new free
registers.
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This actually makes everything slower, but the plan is to have isel add <kill>
flags the way it is already adding <dead> flags. Then LiveVariables can be
removed again.
When ignoring the time spent in LiveVariables, -regalloc=fast is now twice as
fast as -regalloc=local.
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So far this is just a clone of -regalloc=local that has been lobotomized to run
25% faster. It drops the least-recently-used calculations, and is just plain
stupid when it runs out of registers.
The plan is to make this go even faster for -O0 by taking advantage of the short
live intervals in unoptimized code. It should not be necessary to calculate
liveness when most virtual registers are killed 2-3 instructions after they are
born.
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