similar to: [LLVMdev] PBQP crash

> A node should never be put into the conservatively allocatable list if there is a chance of it spilling. I can understand why the logic of NodeMetadata::isConservativelyAllocatable is necessary for the node to be allocatable, but I have not been able to convince myself this is sufficient, especially when the node degree > available registers. Cheers, Arnaud From:

Hi, Sorry for the delay, it has taken some extra time as more than one bug showed up ☺ I continued to look into this with your viewpoint that a node that is conservatively allocatable should never be spilled. The first thing I did was therefore to add some extra code with an assert for this. I believe I then found three bugs and fixed the two: Bug 1: Incorrect transpositions in handleAddEdge()

Hi Arnaud, The conservatively allocatable test is supposed to check two conditions, either of which would be sufficient to make a node allocatable: (1) There exists some register that is not aliased by any register option for any neighbor. This is the "safe row" test. It is straightforward, but likely to fire only rarely. (2) The sum of the maximum number of registers aliased by any

- Re-sending to include llvm-dev. HI Jonas, This is great - thank you very much for your analysis! You're spot on about Bug 1 - the row/column checks are transposed there. I have fixed this in r227628. Regarding Bug 2, as discussed on the other thread I'm going to teach the register allocator to prune single-option vregs so that they never make it into the graph. I haven't had a

Hi, I would like to ask about PBQPs use of InlineSpiller. The code output when using PBQP gets a lot bigger compared to when using RegAllocGreedy. PBQP does not split the live intervals, and a lot more (often redundant) reload instructions are emitted as a result, it seems. I wonder why this is, and if there are any plans to improve on this point? /Jonas Paulsson -------------- next part

Hi, I have worked a little on the PBQP register allocator, and it is quite clear (at least to me) that it is not even a serious alternative to RegAllocGreedy at the moment, due to the poor handling of spilling. As Arnaud wrote below, it is not optimizing spilling at all, but rather just spills anything that does not get an assignment. The result is a lot more spill/reload instructions than

Hi Quentin, Jonas, Splitting fits in with PBQP reasonably well, at least conceptually. The PBQP graph is designed to be mutable, so there is no problem with updating it when splitting. As I see it, there are two logical places to integrate splitting into PBQP: 1) Split during spilling -- If a PBQP solution selects the spill option for a node, rather than spill immediately, split the interval

Both approaches are not exclusive. I would even think it makes sense to have a pre-split pass to prepare the graph, with a global view, and later on use use trySplit (or an equivalent) to handle the local coloring issues. From: Quentin Colombet [mailto:qcolombet at apple.com] Sent: 09 March 2015 23:08 To: Lang Hames Cc: Jonas Paulsson; llvmdev at cs.uiuc.edu; Arnaud De Grandmaison Subject:

Hi, I have encountered a test case where InlineSpiller generates bad code. A register is reloaded but never spilled, and I suspect a bug in InlineSpiller. A register is live over a loop that also have two inner loops. It is not used or defined over the inner loops. It is split into two sibling registers, where one covers just the inner loops interval, which is then spilled. In spill(),

Hi Jonas, Thanks for your patience. After spending some time looking at the additional output you gave me, I agree that your fix is the right one. I was worried that this problem may arise because we were spilling not real user, but in fact what I thought was the problem is an optimization we could do :). See my comments inlined for a few nitpicks before you commit. Thanks again, -Quentin On

Hi Quentin, I have tried to find a test case for an official target, but failed. It seems to be a rare case. To do it, I added the 'else' clause in the following: ... if (VNI->def == OrigVNI->def) { DEBUG(dbgs() << "orig phi value\n"); SVI->second.DefByOrigPHI = true; SVI->second.AllDefsAreReloads = false; propagateSiblingValue(SVI); continue;

> On Jan 19, 2012, at 5:31 AM, Jonas Paulsson wrote: > LLVM would have to be extended with an RegClass/register-attribute 'spillable' > > > What exactly are you proposing? Why can't you do what the PowerPC and Hexagon targets do? Yes, I can move a CR to a GPR and save it to the stack, but due to a very irregular register file this is about 10 times more expensive

Hey all, I've found a bug in either the PBQP register allocator or in VirtRegRewriter. I'm observing this assertion in VirtRegRewriter::rewrite() fail: unsigned VirtReg = MO.getReg(); unsigned PhysReg = VRM->getPhys(VirtReg); ... assert(!MRI->isReserved(PhysReg) && "Reserved register assignment"); Indeed there is a case where

One piece of code I'm writing has a lot of intermediates, and I'm trying to optimize down the number of memory accesses. Here's a snippet from the start of the function, where I think there is some low-hanging fruit: # BB#0: pushq %rbp pushq %r15 pushq %r14 pushq %r13 pushq %r12 pushq %rbx movq %rdx, %rcx movq %rdi, -16(%rsp) # 8-byte Spill movq (%rsi), %rdi movq

On Jan 19, 2012, at 5:31 AM, Jonas Paulsson wrote: > LLVM would have to be extended with an RegClass/register-attribute 'spillable' What exactly are you proposing? Why can't you do what the PowerPC and Hexagon targets do? Spill-free register allocation sounds great, why not do it for all register classes? > , and a register allocator would have to implement register pairing.

Attached is a small patch to allow users of the PBQP allocator to optionally insert a custom pass. I believe it can be usefull to other users of the pbqp. I used it to undo some of the coalescer work, and make sure that I have different virtual registers, inserting a copy if necessary, to build a pair. I noticed an unexpected --- to me at least --- behaviour of the allocator. I have some

Hi, My target has special requirements during register allocation - there is both a need to handle register pairing and to never spill a flag result reg-class (which might happen at -O0 for no obvious reason). Since neither of these issues seems to be supported, I have tried to pre-allocate these registers in the preRA pass. This has resulted in "using undefined physical register"

On Jan 20, 2012, at 6:40 AM, Jonas Paulsson wrote: > > What exactly are you proposing? Why can't you do what the PowerPC and Hexagon targets do? > > Yes, I can move a CR to a GPR and save it to the stack, but due to a very irregular register file this is about 10 times more expensive than saving/restoring an ordinary register. These registers should basically never > have to

Hi Quentin, I have rerun the test case on a recent commit, so the numbers have changed. There are also now a few more basic blocks very small basic blocks in the function, and therefore there are some slight differences. I tried to go back to earlier commits, without success for some reason... This is however very similar, except that there becomes two COPYs back to sibling value after the loop.

Hi All, I finally had a chance to get back to my pbqp trials, now using the 3.0 release. I still hit the same assert : "Attempting to spill already spilled value." This is triggered because in RegAllocPBQP::mapPBQPToRegAlloc, a vreg node is either : - a physical register (problem.isPRegOption(vreg, alloc)), - or a spill (problem.isSpillOption(vreg, alloc)) The problem is that pass