-C llvm-args is parsed inconsistently within LLVM with the arguments llc accepts

[yongqli]

Please see this StackOverflow question.

[nathanross]

Interesting:

rustc -C llvm-args='--list' gives the option list for LLC

you might expect llvm-args would be passed to clang (which has the options described in the stack overflow page), and indeed this seems to be the binary to which these arguments were formerly passed.

edit: but it also doesn't seem to be the case recently, as both stable and nightly seem to pass args to llc, not clang.

[Tyler-Hardin]

I think this may actually be a real thing.

See rust -C llvm-args='-soft-float'.

It throws the error:

rustc: Unknown command line argument '-soft-float'.  Try: 'rustc -help'
rustc: Did you mean '-asan-opt'?`

But llc definitely takes this argument. I've traced the argument parsing to llvm/lib/Support/CommandLine.cpp:CommandLineParser::ParseCommandLineOptions().

And now that I try it, --list doesn't seem to work for me either. I'm running 4c99649.

Btw, I ran into this trying to work around #26749.

[Tyler-Hardin]

Added code to print the argv passed to CommandLineParser::ParseCommandLineOptions(). It prints the args from llvm-args. So I guess the problem is that llc args != whatever arguments that function expects.

[JinShil]

So I guess the problem is that llc args != whatever arguments that function expects.

Yes, that appears to be the case.

The arguments recognized by llc and the arguments recognized by the llvm::cl::ParseCommandLineOptions() API are different. The Rust compiler uses the LLVM API, so it doesn't recognize certain LLVM options even though they may be valid for llc.

Consider the following:

#include "llvm/Support/CommandLine.h"

using namespace llvm;

int main(int argc, char** argv) {
    cl::ParseCommandLineOptions(argc, argv, "");

    return 0;
}

Compile with g++ hello.cc -lLLVM-3.8.1 and execute with a.out -help. It will print the following:

OVERVIEW: 
USAGE: a.out [options]

OPTIONS:

General options:

  -aarch64-neon-syntax                            - Choose style of NEON code to emit from AArch64 backend:
    =generic                                      -   Emit generic NEON assembly
    =apple                                        -   Emit Apple-style NEON assembly
  -bounds-checking-single-trap                    - Use one trap block per function
  -color                                          - use colored syntax highlighting (default=autodetect)
  -cppfname=<function name>                       - Specify the name of the generated function
  -cppfor=<string>                                - Specify the name of the thing to generate
  -cppgen                                         - Choose what kind of output to generate
    =program                                      -   Generate a complete program
    =module                                       -   Generate a module definition
    =contents                                     -   Generate contents of a module
    =function                                     -   Generate a function definition
    =functions                                    -   Generate all function definitions
    =inline                                       -   Generate an inline function
    =variable                                     -   Generate a variable definition
    =type                                         -   Generate a type definition
  -disable-spill-fusing                           - Disable fusing of spill code into instructions
  -enable-implicit-null-checks                    - Fold null checks into faulting memory operations
  -enable-load-pre                                - 
  -enable-objc-arc-opts                           - enable/disable all ARC Optimizations
  -enable-scoped-noalias                          - 
  -enable-tbaa                                    - 
  -exhaustive-register-search                     - Exhaustive Search for registers bypassing the depth and interference cutoffs of last chance recoloring
  -filter-print-funcs=<function names>            - Only print IR for functions whose name match this for all print-[before|after][-all] options
  -gpsize=<uint>                                  - Global Pointer Addressing Size.  The default size is 8.
  -imp-null-check-page-size=<uint>                - The page size of the target in bytes
  -internalize-public-api-file=<filename>         - A file containing list of symbol names to preserve
  -internalize-public-api-list=<list>             - A list of symbol names to preserve
  -join-liveintervals                             - Coalesce copies (default=true)
  -limit-float-precision=<uint>                   - Generate low-precision inline sequences for some float libcalls
  -merror-missing-parenthesis                     - Error for missing parenthesis around predicate registers
  -merror-noncontigious-register                  - Error for register names that aren't contigious
  -mfuture-regs                                   - Enable future registers
  -mips16-constant-islands                        - Enable mips16 constant islands.
  -mips16-hard-float                              - Enable mips16 hard float.
  -mno-compound                                   - Disable looking for compound instructions for Hexagon
  -mno-ldc1-sdc1                                  - Expand double precision loads and stores to their single precision counterparts
  -mno-pairing                                    - Disable looking for duplex instructions for Hexagon
  -mwarn-missing-parenthesis                      - Warn for missing parenthesis around predicate registers
  -mwarn-noncontigious-register                   - Warn for register names that arent contigious
  -mwarn-sign-mismatch                            - Warn for mismatching a signed and unsigned value
  -no-discriminators                              - Disable generation of discriminator information.
  -nvptx-sched4reg                                - NVPTX Specific: schedule for register pressue
  -print-after-all                                - Print IR after each pass
  -print-before-all                               - Print IR before each pass
  -print-machineinstrs=<pass-name>                - Print machine instrs
  -rdf-dump                                       - 
  -rdf-limit=<uint>                               - 
  -regalloc                                       - Register allocator to use
    =default                                      -   pick register allocator based on -O option
    =basic                                        -   basic register allocator
    =fast                                         -   fast register allocator
    =greedy                                       -   greedy register allocator
    =pbqp                                         -   PBQP register allocator
  -rewrite-map-file=<filename>                    - Symbol Rewrite Map
  -rng-seed=<seed>                                - Seed for the random number generator
  -sample-profile-check-record-coverage=<N>       - Emit a warning if less than N% of records in the input profile are matched to the IR.
  -sample-profile-check-sample-coverage=<N>       - Emit a warning if less than N% of samples in the input profile are matched to the IR.
  -sample-profile-global-cold-threshold=<N>       - Top-level functions that account for less than N% of all samples collected in the profile, will be marked as cold for the inliner to consider.
  -sample-profile-global-hot-threshold=<N>        - Top-level functions that account for more than N% of all samples collected in the profile, will be marked as hot for the inliner to consider.
  -sample-profile-inline-hot-threshold=<N>        - Inlined functions that account for more than N% of all samples collected in the parent function, will be inlined again.
  -sample-profile-max-propagate-iterations=<uint> - Maximum number of iterations to go through when propagating sample block/edge weights through the CFG.
  -stackmap-version=<int>                         - Specify the stackmap encoding version (default = 1)
  -stats                                          - Enable statistics output from program (available with Asserts)
  -summary-file=<string>                          - The summary file to use for function importing.
  -time-passes                                    - Time each pass, printing elapsed time for each on exit
  -verify-debug-info                              - 
  -verify-dom-info                                - Verify dominator info (time consuming)
  -verify-loop-info                               - Verify loop info (time consuming)
  -verify-regalloc                                - Verify during register allocation
  -verify-region-info                             - Verify region info (time consuming)
  -verify-scev                                    - Verify ScalarEvolution's backedge taken counts (slow)
  -x86-asm-syntax                                 - Choose style of code to emit from X86 backend:
    =att                                          -   Emit AT&T-style assembly
    =intel                                        -   Emit Intel-style assembly

Generic Options:

  -help                                           - Display available options (-help-hidden for more)
  -help-list                                      - Display list of available options (-help-list-hidden for more)
  -version                                        - Display the version of this program

Compare that with the output for llc -help (an exercise left to the reader). I need the -function-sections and -data-sections for my target, as they don't appear to be enabled by default.

I don't know what the solution is here. Is it something for LLVM to solve, or something for rustc to solve?

[Mark-Simulacrum]

I'm leaving this open because it's not clear to me how a user should pass arguments directly to llc if they want to; perhaps that's not possible or even something we want. @rust-lang/compiler: Could you comment on whether we think it's a good idea to allow passing arguments into llc directly? Do we run llc at all? If we don't then it seems that this is not a bug.

[Mark-Simulacrum]

Nominating to decide whether we want to expose llc or not (and if we can -- I don't know that we run it directly today). If not, we should close.

[nikomatsakis]

I'm moving this to T-dev-tools; it seems like they've put some energy into how to think about the stability of llvm-args #40063 as well.

[nrc]

cc @alexcrichton @vadimcn this seems like something that would be a tooling/platform team issue if that team existed.

[nrc]

Briefly discussed at the dev-tools meeting: verdict was that this did not seem urgent to address.

[jethrogb]

Related #68059

[sethp]

Leaving this here in case it's helpful to anyone who comes across this issue: I was able to get what appears to be the full list of the arguments LLVM will accept by way of -C llvm-args with:

rustc -C llvm-args='--help-list-hidden'

Caution: sharp edges abound.

[nikic]

Exposing these flags would be a matter of using RegisterCodeGenFlags, if we really want to.