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

Kyriakos Georgiou via cfe-dev cfe-dev at lists.llvm.org
Mon Dec 4 04:31:33 PST 2017


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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