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The results of llvm ir and c are not the same

chaojibendan opened this issue · comments

commented 
bambu --top-fname=calc_sha_256 ../sha-256.c --compiler=I386_CLANG12 --simulate --simulator=ICARUS --generate-tb=../test.xml -O3

C's result is :

  Total number of flip-flops in function calc_sha_256: 3649
  Total cycles             : 356 cycles
  Number of executions     : 1
  Average execution        : 356 cycles

clang-12 -S -emit-llvm sha-256.c -o sha-256.ll -O3
cd test
bambu --top-fname=calc_sha_256 ../sha-256.ll --compiler=I386_CLANG12 --simulate --simulator=ICARUS --generate-tb=../test_ll.xml

LLVM IR's result is :

  Total number of flip-flops in function calc_sha_256: 4209
  Total cycles             : 365 cycles
  Number of executions     : 1
  Average execution        : 365 cycles

Is bambu using extra compilation options by default?

commented

Yes, Bambu uses extra options. If you pass —no-clean and check the scripts in panda-temp directory you will see how we use clang or gcc

commented

Yes, Bambu uses extra options. If you pass —no-clean and check the scripts in panda-temp directory you will see how we use clang or gcc

I pass —no-clean and see many scripts in panda-temp directory. shell_script_8 and shell_script_10 look relevant to my question. I compare .c and .ll's shell_script and find that they are identical include extra options , except for the filename(.c .ll).
Then I check their __gcc_output:
C:

clang: warning: optimization flag '-ffloat-store' is not supported [-Wignored-optimization-argument]
clang: warning: optimization flag '-fexcess-precision=standard' is not supported [-Wignored-optimization-argument]
clang: warning: optimization flag '-ffloat-store' is not supported [-Wignored-optimization-argument]
clang: warning: optimization flag '-fexcess-precision=standard' is not supported [-Wignored-optimization-argument]

LLVM IR:

clang: warning: optimization flag '-ffloat-store' is not supported [-Wignored-optimization-argument]
clang: warning: optimization flag '-ffloat-store' is not supported [-Wignored-optimization-argument]
warning: overriding the module target triple with i386-pc-linux-gnu [-Woverride-module]
1 warning generated.

So when I use clang-12 -S -emit-llvm sha-256.c -o sha-256.ll -O3, I lost some flags in shell_script that can only be used in the pass(C to LLVM IR). This may account for the difference in results.

commented

thank you!