How to prevent from generating a huge constant table ? about c2goasm HOT 6 CLOSED

I modify the asm code to this manually

.LCPI1_0:
	.quad	-9223372036854775808    # 0x8000000000000000
        .quad	-9223372036854775808    # 0x8000000000000000
        .quad	-9223372036854775808    # 0x8000000000000000
        .quad	-9223372036854775808    # 0x8000000000000000
	.section	.rodata,"a",@progbits
.LCPI1_1:
	.long	0                       # 0x0
	.long	2                       # 0x2
	.long	4                       # 0x4
	.long	6                       # 0x6
	.zero	4
	.zero	4
	.zero	4
	.zero	4
	.text
	.globl	sample_max_avx2

and it works. Is it a correct way and a safely way?

from c2goasm.

Yes, that looks correct and should be ok.

from c2goasm.

@fwessels thanks for your reply.
Does clang have a flag to enable/disable move the local variables to constant? I doesnot find a relevant flag and I does not know clang well .

from c2goasm.

@iRita Sorry for responding late to your question. Looks like your question got answered on https://stackoverflow.com/questions/50126786/how-to-prevent-clang-llvm-compile-local-variables-to-global . Did you get a chance to test the suggestion? Also, clang has lots of flags that the result can be different with different combinations of flags. so, please try with the options given here https://clang.llvm.org/docs/ClangCommandLineReference.html

from c2goasm.

Hi @kannappanr,
If using -Os instead of -O3, the clang generate asm code without AVX2 instructions. It is not we wanted, we want the asm codes which is optimized for performance.

There are too many optimizations in -O3, and which one affects the move local variables to constant should be tested.

I do not find a quickly and easy way to do the test. Do I must generate the asm code by a hard way (llvm->opt->llc) ？

https://laure.gonnord.org/pro/teaching/CAP1718_ENSL/llvm_fernando.pdf

from c2goasm.

Hello @lrita , I am afraid you will need to do some testing on your own, clang has a ton of options and we only know the (bare) basics of it.

Going the opt / llc way is certainly another option, but these tools have their own set of flags etc so you will need to study this.

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