[llvm-dev] Dynamic Spill and Instruction Counts via LLVM

[Quentin Colombet via llvm-dev]

Hi Armand,

You can print the frequency of the blocks at the machine level.
If that frequency comes from pgo, these numbers should be accurate.

Take a look at -view-machine-block-freq-propagation-dags=fraction for instance.

Then, you should be able to just use the assembly file annotation like you were doing.

Cheers,
-Quentin

> On Nov 30, 2020, at 3:30 PM, Armand Behroozi via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> 
> Hello,
> 
> How would people recommend using LLVM (if at all) to get the dynamic spill/reload count and total number of instructions for a program? Speed and efficiency are unimportant. I just want it to be correct.
> 
> I'm using a debug build of LLVM 12 "clang version 12.0.0" on an x86_64-unknown-linux-gnu cpu.
> 
> 
> What I've tried:
> I thought I'd use llvm's pgo to get asm BB counts and then multiply by the number of static spills and reloads in each asm BB. 
> 
> LLC annotates lines in the assembly file that result in spills and reloads i.e.:
> movq %rax, -24(%rsp)                 # 8-byte Spill
> movq -16(%rsp), %rax                 # 8-byte Reload
> 
> I couldn't find a way to instrument asm BBs w/ pgo using LLC, so I'm doing it at the IR level like this:
> clang -emit-llvm -gline-tables-only -O3 file1.c file1.bc
> opt -pgo-instr-gen -instrprof  file1.bc -o file1.prof.bc  
> clang -fprofile-instr-generate file1.prof.bc file2.prof.bc etc.
> -run it-
> llvm-profdata merge -output=pgo.profdata default.profraw
> opt -pgo-instr-use -pgo-test-profile-file=pgo.profdata -load mypass file.bc
> 
> Where "mypass" iterates through all the IR BBs and prints out the label and block frequency info.
> 
> I've quickly realized that asm BBs don't directly correspond to IR BBs b/c the CFG changes during code generation. Although I've managed to make a mostly correct mapping between IR BBs and asm BBs using their debug labels, it's been kind of messy. For example:
> There is the odd situation where I'll find 2 asm BBs with the same debug label w/ pgo counters to different memory addresses.
> 
> There are also other complications where the pgo CFG is different from the original CFG, so I'm not convinced this is the correct way to go about it.
> 
> 
> What I thought I'd try next
> I wanted to write a MachineFunctionPass that gets a BB's static spill/reload count and then inserts a function call to keep track of its execution frequency. The pass would be run after all CFG changes have been made. There's a comment in X86TargetMachine.cpp that says: 
> "  
> // The X86 Speculative Execution Pass must run after all control
> // flow graph modifying passes.
> "
> This is in void X86PassConfig::addPreEmitPass2(), and I was going to add my pass here. Since the function call would clobber registers, I was going to either re-run register allocation or callee save all the registers the function would use.
> 
> Any help/feedback would be appreciated.
> 
> Thank you.
> 
> Regards,
> 
> Armand Behroozi
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