[llvm-dev] flags to reproduce clang -O3 with opt -O3

Thu Jul 2 18:34:28 PDT 2020

On Thu, Jul 2, 2020 at 2:28 PM Chad Verbowski via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Hello,
>
> I've been trying to figure out how to reproduce the results of a single
> clang -O3 compilation to a binary with a multi-step process using opt.
>
> Specifically I have:
>
> clang -O3 foo.c -o foo.exe
>
>
> which I want to replicate with:
>
> clang -O0 -c -emit-llvm foo.c
>
>
Using O0 will mark every function in the IR with "optnone" which prevents
`opt` from optimizing it. I'd try `clang -O3  -Xclang
-disable-llvm-passes -c -emit-llvm foo.c`

> opt -O3 foo.bc -o foo_o.bc
> clang foo_o.bc -o foo.exe
>
>
This last step won't enable optimizations in the backend, you likely should
try `llc -O3` instead.

Best,

-- 
Mehdi

>
> Any hints / suggestions on what additional flags I need to produce the
> same binary are greatly appreciated!
>
> *What I've tried:*
> I've been reading the archives, and found this
> <http://lists.llvm.org/pipermail/llvm-dev/2017-September/117144.html>,
> which suggests dumping the pass arguments using:
>
> clang -mllvm -debug-pass=Structure -O3 foo.c -o foo.exe
>
> and comparing with:
>
> clang -mllvm -debug-pass=Structure -O0 -c -emit-llvm foo.c
> opt -debug-pass=Structure -O3 foo.bc -o foo_o.bc
> clang -mllvm -debug-pass=Structure foo_o.bc -o foo.exe
>
>
> The first has 30 "Pass Argument" statements though only these 5 are
> distinct. Across these 5 there are 190 distinct flags.  The multi-step
> compilation has only 140 distinct flags. Comparing the flags, 18 from the
> multi-step are missing in the 1pass, and 67 from 1pass are missing in the
> multistep.
>
> These appear to be opt flags, since they cause an error when trying to use
> them with clang (e.g. -x86-fixup-LEAs) and when used with opt causes a
> crash with stack dump and request to submit a bug report. Others like
> -attributor appear to work with opt.
>
> I'm currently blindly trying to add the 67 different flags to the opt step
> to see which work, and hopefully that subset will produce the same result
> as clang -O3.
>
> It seems like there must be an easier / more exact way of getting the opt
> -O3 multi-step to match the clang -O3 result.
>
> Any thoughts or insights are appreciated. Below is a sorted list of the
> flags missing from each for completeness.
>
> not contained in 1pass O3 (count=18)
>
> -aa-scalar-evolution
>
> -always-inline
>
> -callsite-splitting
>
> -inject-tli-mappings
>
> -ipsccp
>
> -jump-threading-correlated-propagation
>
> -livedebugvalues
>
> -loops-loop-simplify
>
> -memdep-lazy-branch-prob
>
> -openmpopt
>
> -opt-remark-emitter-instcombine
>
> -regallocfast
>
> -speculative-execution
>
> -stackmap-liveness
>
> -tbaa-scoped-noalias
>
> -vector-combine
>
> -verify
>
> -write-bitcode
>
> not contained in multi O3 (count=67)
>
> -attributor
>
> -block-freq-loop-simplify
>
> -branch-folder
>
> -break-false-deps
>
> -callsite-splitting-ipsccp
>
> -codegenprepare
>
> -consthoist
>
> -dead-mi-elimination
>
> -detect-dead-lanes
>
> -early-ifcvt
>
> -early-machinelicm
>
> -early-tailduplication
>
> -expandmemcmp
>
> -greedy
>
> -interleaved-access
>
> -iv-users
>
> -lazy-block-freq-opt-remark-emitter
>
> -livedebugvars
>
> -liveintervals
>
> -liveregmatrix
>
> -livestacks
>
> -livevars
>
> -loop-reduce
>
> -loop-simplify-lcssa-verification
>
> -lrshrink
>
> -machine-block-freq
>
> -machine-combiner
>
> -machine-cp
>
> -machine-cse
>
> -machinedomtree-machine-loops
>
> -machinelicm
>
> -machine-loops
>
> -machinepostdomtree
>
> -machinepostdomtree-block-placement
>
> -machine-scheduler
>
> -machine-sink
>
> -machine-trace-metrics
>
> -mergeicmps
>
> -objc-arc-contract
>
> -opt-phis
>
> -partially-inline-libcalls
>
> -peephole-opt
>
> -postra-machine-sink
>
> -post-RA-sched
>
> -processimpdefs
>
> -reaching-deps-analysis
>
> -rename-independent-subregs
>
> -shrink-wrap
>
> -simple-register-coalescing
>
> -slotindexes
>
> -spill-code-placement
>
> -stack-coloring
>
> -stackmap-liveness-livedebugvalues
>
> -stack-slot-coloring
>
> -tailduplication
>
> -unreachable-mbb-elimination
>
> -virtregmap
>
> -virtregrewriter
>
> -x86-avoid-SFB
>
> -x86-cf-opt
>
> -x86-cmov-conversion
>
> -x86-domain-reassignment
>
> -x86-evex-to-vex-compress
>
> -x86-execution-domain-fix
>
> -x86-fixup-bw-insts
>
> -x86-fixup-LEAs
>
> -x86-optimize-LEAs
>
>
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