similar to: Relationship between clang, opt and llc

> On Apr 10, 2017, at 5:21 PM, Craig Topper via llvm-dev <llvm-dev at lists.llvm.org> wrote: > > clang -O0 does not disable all optimization passes modify the IR.; In fact it causes most functions to get tagged with noinline to prevent inlinining It also disable lifetime instrinsics emission and TBAA, etc. > > What you really need to do is > > clang -O3 -c

It's really nice of you pointing out the -Xclang option, it makes things much easier. I really appreciate your help! Best, Peizhao On Mon, Apr 10, 2017 at 10:12 PM, Mehdi Amini <mehdi.amini at apple.com> wrote: > > On Apr 10, 2017, at 5:21 PM, Craig Topper via llvm-dev < > llvm-dev at lists.llvm.org> wrote: > > clang -O0 does not disable all optimization passes

I tried the following on LULESH1.0 serial version ( https://codesign.llnl.gov/lulesh/LULESH.cc) 1. clang++ -O3 LULESH.cc; ./a.out 20 Runtime: 9.487353 second 2. clang++ -O0 -Xclang -disable-llvm-passes -c -emit-llvm -o a.bc LULESH.cc; opt -O3 a.bc -o b.bc; llc -O3 -filetype=obj b.bc -o b.o ; clang++ b.o -o b.out; ./b.out 20 Runtime: 24.15 seconds 3. clang++ -O3 -Xclang -disable-llvm-passes -c

If you pass -O0 to clang, most functions will be tagged with an optnone function attribute that will prevent opt and llc even if you pass -O3 to opt and llc. This is the mostly likely cause for the slow down in 2. You can disable the optnone function attribute behavior by passing "-Xclang -disable-O0-optnone" to clang ~Craig On Fri, Jan 5, 2018 at 1:19 PM, toddy wang via llvm-dev <

O0 didn't start applying optnone until r304127 in May 2017 which is after the 4.0 family was branched. So only 5.0, 6.0, and trunk have that behavior. Commit message copied below Author: Mehdi Amini <joker.eph at gmail.com> Date: Mon May 29 05:38:20 2017 +0000 IRGen: Add optnone attribute on function during O0 Amongst other, this will help LTO to correctly handle/honor

Craig, thanks a lot! I'm actually confused by clang optimization flags. If I run clang -help, it will show many optimizations (denoted as set A) and non-optimization options (denoted as set B). If I run llvm-as < /dev/null | opt -O0/1/2/3 -disable-output -debug-pass=Arguments, it also shows many optimization flags (denote as set C). There are many options in set C while not in set A,

@Zhaopei, thanks for the clarification. @Craig and @Michael, for clang 4.0.1, -Xclang -disable-O0-optnone gives the following error message. From which version -disable-O0-optnone gets supported? [twang15 at c89 temp]$ clang++ -O0 -Xclang -disable-O0-optnone -Xclang -disable-llvm-passes -c -emit-llvm -o a.bc LULESH.cc error: unknown argument: '-disable-O0-optnone' [twang15 at c89

I don't think "clang -help" prints options about optimizations. Clang itself doesn't have direct support for fine grained optimization control. Just the flag for levels -O0/-O1/-O2/-O3. This is intended to be simple and sufficient interface for most users who just want to compile their code. So I don't think there's a way to pass just -dse to clang. opt on the other hand

Hi, "SetC" options are LLVM cl::opt options, they are intended for LLVM developer and experimentations. If a settings is intended to be used as a public API, there is usually a programmatic way of setting it in LLVM. "SetA" is what clang as a C++ compiler exposes to the end-user. Internally clang will (most of the time) use one or multiple LLVM APIs to propagate a settings.

What I am trying is to compile a program with different sets of optimization flags. If there is no fine-grained control over clang optimization flags, it would be impossible to achieve what I intend. Although there is fine-grained control via opt, for a large-scale projects, clang-opt-llc pipeline may not be a drop-in solution. On Fri, Jan 5, 2018 at 10:00 PM, Craig Topper <craig.topper at

On Jan 5, 2018 11:30 PM, "toddy wang via llvm-dev" <llvm-dev at lists.llvm.org> wrote: What I am trying is to compile a program with different sets of optimization flags. If there is no fine-grained control over clang optimization flags, it would be impossible to achieve what I intend. LLD has -lto-newpm-passes (and the corresponding -lto-newpm-aa-pipeline) which allows you to

After build LLVM5.0, I found that clang-5.0 is extremely slow. Even it is built with -DCMAKE_BUILD_TYPE=Release For building LULESH.cc, it gets stucked at linkage stage. I build it as instructed from here https://github.com/flang-compiler/flang Maybe I should submit a bug. On Fri, Jan 5, 2018 at 8:41 PM, toddy wang <wenwangtoddy at gmail.com> wrote: > Craig, thanks a lot! > >

@Craig and @Michael After installing clang-5.0 (download from http://releases.llvm.org, does not have Flang build's slowdown mention above), 1. clang++ -O0 -Xclang -disable-O0-optnone -Xclang -disable-llvm-passes -c -emit-llvm -o a.bc LULESH.cc; opt -O3 a.bc -o b.bc; llc -O3 -filetype=obj b.bc -o b.o ; clang++ b.o -o b.out; ./b.out 20 runtime: 2.354069e+01 2. clang++ -O1 -Xclang

Hi Toddy, You can achieve what you're looking for with a pipeline based on `clang -Ox` + `opt -Ox` + `llc -Ox` (or lld instead of llc), but this won't be guarantee'd to be well supported across releases of the compiler. Otherwise, if there are some performance-releated (or not...) command line options you think clang is missing / would benefit, I invite you to propose adding them to

Thanks a lot, Mehdi. For GCC, there are around 190 optimization flags exposed as command-line options. For Clang/LLVM, the number is 40, and many important optimization parameters are not exposed at all, such as loop unrolling factor, inline function size parameters. I understand there is very different idea for whether or not expose many flags to end-user. Personally, I believe it is a

@Sean, do you mean llc ? For llc 4.0 and llc 5.0, I cannot find -lto-newpm-passes option, is it a hidden one? On Sat, Jan 6, 2018 at 7:37 PM, Sean Silva <chisophugis at gmail.com> wrote: > > > On Jan 5, 2018 11:30 PM, "toddy wang via llvm-dev" < > llvm-dev at lists.llvm.org> wrote: > > What I am trying is to compile a program with different sets of >

Thanks a lot, it is clear to me now. BTW, for Clang's slowdown, I submit an issue here: https://github.com/flang-compiler/flang/issues/356 I have no idea about the root cause. Maybe due to debug symbols. But, I already use -DCMAKE_BUILD_TYPE=Release. Anyway, I believe there is a bug somewhere. On Sat, Jan 6, 2018 at 3:43 PM, Craig Topper <craig.topper at gmail.com> wrote: >

For the types of things that you are looking for, you may just want to try a bunch of -mllvm options. You can tune inlining and unrolling threshold like that, for example. On Jan 7, 2018 10:33 PM, "toddy wang via llvm-dev" <llvm-dev at lists.llvm.org> wrote: > Hi Mehdi, > > Now we have 5 pipelines. (In addition to the first 3, which I have > described in detail above,

-disable-O0-optnone has no effect with anything other than -O0. -O0 being passed to clang also causes all functions to be marked noinline. I don't know if there is a command line option to turn that off. I recommend passing "-O1 -Xclang -disable-llvm-passes" to clang. Passing -O0 very specifically means disable optimizations. ~Craig On Sat, Jan 6, 2018 at 12:25 PM, toddy wang

No, I meant LLD, the LLVM linker. This option for LLD is relevant for exploring different pass pipelines for link time optimization. It is essentially equivalent to the -passes flag for 'opt'. Such a flag doesn't make much sense for 'llc' because llc mostly runs backend passes, which are much more difficult to construct custom pipelines for (backend passes are often required