llvm dev - Dec 2010 - [LLVMdev] opinions on turning on encoding info by default in -S

On targets that fully support MC for encoding (today that means X86, hopefully
ARM will be done soon), llc has a "-show-mc-encoding" option that can
be used to dump out the encodings for every instruction.  If you're using
clang, this can be accessed with the (intentionally) hidden -mllvm option like
so:

$ clang t.c -S -o - -mllvm -show-mc-encoding -mkernel -O3 -fomit-frame-pointer
	.section	__TEXT,__text,regular,pure_instructions
	.globl	_tf_bH
	.align	4, 0x90
_tf_bH:                                 ## @tf_bH
## BB#0:                                ## %entry
                                        ## kill: RSI<def> ESI<kill>
	movq	_last_tf_arg_u at GOTPCREL(%rip), %rax ## encoding:
[0x48,0x8b,0x05,A,A,A,A]
                                        ##   fixup A - offset: 3, value:
_last_tf_arg_u at GOTPCREL-4, kind: reloc_riprel_4byte_movq_load
	movq	%rsi, (%rax)            ## encoding: [0x48,0x89,0x30]
	imull	$43691, %esi, %eax      ## encoding: [0x69,0xc6,0xab,0xaa,0x00,0x00]
                                        ## imm = 0xAAAB
	shrl	$17, %eax               ## encoding: [0xc1,0xe8,0x11]
	ret                             ## encoding: [0xc3]

	.comm	_last_tf_arg_u,8,3      ## @last_tf_arg_u
                                        ## @last_tf_arg_u

.subsections_via_symbols



Relatively recently, we turned on verbose-asm output by default in clang for -S
output on targets that support the integrated assembler (which is almost all of
them on x86 now).  Does anyone have a reason to disable the encoding
information?  On the one hand it is fairly noisy, on the other hand, it is very
useful and is helpful when tuning code for code size.  In any case, this
doesn't affect normal compile times that use "-c".

Thoughts?

-Chris

On Dec 14, 2010, at 9:37 PM, Chris Lattner wrote:
> On targets that fully support MC for encoding (today that means X86,
hopefully ARM will be done soon), llc has a "-show-mc-encoding" option
that can be used to dump out the encodings for every instruction.  If you're
using clang, this can be accessed with the (intentionally) hidden -mllvm option
like so:
> 
> $ clang t.c -S -o - -mllvm -show-mc-encoding -mkernel -O3
-fomit-frame-pointer
> 	.section	__TEXT,__text,regular,pure_instructions
> 	.globl	_tf_bH
> 	.align	4, 0x90
> _tf_bH:                                 ## @tf_bH
> ## BB#0:                                ## %entry
>                                        ## kill: RSI<def>
ESI<kill>
> 	movq	_last_tf_arg_u at GOTPCREL(%rip), %rax ## encoding:
[0x48,0x8b,0x05,A,A,A,A]
>                                        ##   fixup A - offset: 3, value:
_last_tf_arg_u at GOTPCREL-4, kind: reloc_riprel_4byte_movq_load
> 	movq	%rsi, (%rax)            ## encoding: [0x48,0x89,0x30]
> 	imull	$43691, %esi, %eax      ## encoding: [0x69,0xc6,0xab,0xaa,0x00,0x00]
>                                        ## imm = 0xAAAB
> 	shrl	$17, %eax               ## encoding: [0xc1,0xe8,0x11]
> 	ret                             ## encoding: [0xc3]
> 
> 	.comm	_last_tf_arg_u,8,3      ## @last_tf_arg_u
>                                        ## @last_tf_arg_u
> 
> .subsections_via_symbols
> 
> 
> 
> Relatively recently, we turned on verbose-asm output by default in clang
for -S output on targets that support the integrated assembler (which is almost
all of them on x86 now).  Does anyone have a reason to disable the encoding
information?  On the one hand it is fairly noisy, on the other hand, it is very
useful and is helpful when tuning code for code size.  In any case, this
doesn't affect normal compile times that use "-c".
> 
> Thoughts?
It's too noisy for my taste, and the situations where that info would be
helpful to me are pretty rare.
I'd rather have it disabled by default.

On Dec 14, 2010, at 9:37 PM, Chris Lattner wrote:
> On targets that fully support MC for encoding (today that means X86,
hopefully ARM will be done soon), llc has a "-show-mc-encoding" option
that can be used to dump out the encodings for every instruction.  If you're
using clang, this can be accessed with the (intentionally) hidden -mllvm option
like so:
> 
> $ clang t.c -S -o - -mllvm -show-mc-encoding -mkernel -O3
-fomit-frame-pointer
> 	.section	__TEXT,__text,regular,pure_instructions
> 	.globl	_tf_bH
> 	.align	4, 0x90
> _tf_bH:                                 ## @tf_bH
> ## BB#0:                                ## %entry
>                                        ## kill: RSI<def>
ESI<kill>
> 	movq	_last_tf_arg_u at GOTPCREL(%rip), %rax ## encoding:
[0x48,0x8b,0x05,A,A,A,A]
>                                        ##   fixup A - offset: 3, value:
_last_tf_arg_u at GOTPCREL-4, kind: reloc_riprel_4byte_movq_load
> 	movq	%rsi, (%rax)            ## encoding: [0x48,0x89,0x30]
> 	imull	$43691, %esi, %eax      ## encoding: [0x69,0xc6,0xab,0xaa,0x00,0x00]
>                                        ## imm = 0xAAAB
> 	shrl	$17, %eax               ## encoding: [0xc1,0xe8,0x11]
> 	ret                             ## encoding: [0xc3]
> 
> 	.comm	_last_tf_arg_u,8,3      ## @last_tf_arg_u
>                                        ## @last_tf_arg_u
> 
> .subsections_via_symbols
> 
> 
> 
> Relatively recently, we turned on verbose-asm output by default in clang
for -S output on targets that support the integrated assembler (which is almost
all of them on x86 now).  Does anyone have a reason to disable the encoding
information?  On the one hand it is fairly noisy, on the other hand, it is very
useful and is helpful when tuning code for code size.  In any case, this
doesn't affect normal compile times that use "-c".
This cries out for different levels of verbosity. How about printing the
encoding information for -verbose-asm=2 (or higher)?

Evan
> 
> Thoughts?
> 
> -Chris
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev

On Thursday, 16 December 2010,
> This cries out for different levels of verbosity. How about printing the
encoding information for -verbose-asm=2 (or higher)?
I concur, the current verbose level is enough for all my needs, now.
If/when I start fiddling with MC, I might find that useful, but a
special flag would do just fine.

Cheers,
Renato

-- 
cheers,
--renato

http://systemcall.org/

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