[cfe-dev] Opt seems to report fewer stats when splitting the compilation to the several stages, rather than using clang

[Craig Topper via cfe-dev]

You need to invoke clang with -O2 -Xclang -disable-llvm-optzns instead of
-O0

-O0 implies to clang to mark all functions as optnone in the generated IR

On Mon, Dec 4, 2017 at 4:31 AM Kyriakos Georgiou via cfe-dev <
cfe-dev at lists.llvm.org> wrote:

> Hi,
>
>
> I am trying to recreate the compilation of using clang and -O2 by
> splitting the compilation to the several stages using the several
> llvm tools (clang as front-end, opt, llc..) but the optimiser seems to
> behave differently if called individually than when called by clang. I am
> using the latest llvm version. So I am calling the front end using:
>
>
> clang  -mllvm -stats -mllvm -debug-pass=Structure -target arm-none-eabi
> -isystem
> /usr/local/lpcxpresso_8.2.2_650/lpcxpresso/tools/arm-none-eabi/include
> -DHAVE_CONFIG_H -I. -I../.. -Wall -I ../../i^Clude -I
> ../../config/arm/boards/lpcxpresso_1769_xpresso/ -I
> ../../config/arm/chips/lpc175x_6x/ "-DCALIB_SCALE=0" --static -mthumb
> -mcpu=cortex-m3 -D__USE_LPCOPEN -D__CODE_RED -D__REDLIB__ -DCHIP_LPC175X_6X
> -DCORE_M3 -ffunction-sections -fdata-sections -O2 -c -emit-llvm  -o
> adpcm_optimized.bc adpcm.c
>
>
> this also gives me the optimizer statistics and the structure of calling
> the passes.
>
> then I am using
>
>
> clang  -mllvm -stats -mllvm -debug-pass=Structure -target arm-none-eabi
> -isystem
> /usr/local/lpcxpresso_8.2.2_650/lpcxpresso/tools/arm-none-eabi/include
> -DHAVE_CONFIG_H -I. -I../.. -Wall -I ../../i^Clude -I
> ../../config/arm/boards/lpcxpresso_1769_xpresso/ -I
> ../../config/arm/chips/lpc175x_6x/ "-DCALIB_SCALE=0" --static -mthumb
> -mcpu=cortex-m3 -D__USE_LPCOPEN -D__CODE_RED -D__REDLIB__ -DCHIP_LPC175X_6X
> -DCORE_M3 -ffunction-sections -fdata-sections -O0 -c -emit-llvm  -o
> adpcm_no_opt.bc adpcm.c
>
> and then
>
> opt -stats -O2 -S  -debug-pass=Structure adpcm_no_opt.bc -o adpcm.ll
>
> I was expecting the stats coming out of the opt call to be the same as
> calling clang with -O2. Instead they are very little:
>
>  18 basicaa          - Number of times a GEP is decomposed
>
>   1 cgscc-passmgr    - Maximum CGSCCPassMgr iterations on one SCC
>
> 104 globalopt        - Number of globals marked unnamed_addr
>
>   6 globalsmodref-aa - Number of global vars without address taken
>
>
> Am I doing something wrong or missing something? The Structure of calling
> the passes is still the same,  and also the  emitted code from the clang
> with -O2 and the opt with -O2 seems to be the same. So why is the optimiser
> called individually not reporting the full stats?
>
>
> Thank you,
>
> Kyriakos
>
>
> _______________________________________________
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> cfe-dev at lists.llvm.org
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>
-- 
~Craig
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