[PATCH] D34689: [LLD][ELF] Pre-create ThunkSections at Target specific intervals

[Peter Smith via Phabricator via llvm-commits]

peter.smith added a comment.

In https://reviews.llvm.org/D34689#811237, @ikudrin wrote:

> It is possible that a precreated thunk section stays empty in the first pass, but is used in the next pass. In that case, it isn't added into ISR in `ThunkCreator::mergeThunks()`. For example:
>
>   $ cat a.s
>   	.global _start, foo
>   .section .text.start,"ax",%progbits
>   _start:
>   	bl _start
>   .section .text.dummy1,"ax",%progbits
>   .space 0xfffffe
>   .section .text.foo,"ax",%progbits
>   foo:
>   	bl _start
>   $ llvm-mc -filetype=obj -triple=thumbv7a-none-linux-gnueabi a.s -o a.o
>   $ ld.lld a.o -o a.out
>   $ objdump -d a.out
>   a.out:     file format elf32-littlearm
>  
>  
>   Disassembly of section .text:
>  
>   00011000 <_start>:
>      11000:	f7ff effe 	blx	11000 <_start>
>  
>   00011004 <__Thumbv7ABSLongThunk__start>:
>      11004:	f241 0c00 	movw	ip, #4096	; 0x1000
>      11008:	f2c0 0c01 	movt	ip, #1
>      1100c:	4760      	bx	ip
>   	...
>  
>   01011002 <__Thumbv7ABSLongThunk__start>:
>   	...
>  
>   0101100c <foo>:
>    101100c:	f7ff fff9 	bl	1011002 <__Thumbv7ABSLongThunk__start>
>
>
> By the way, this sample shows one additional issue: redundant thunks are not removed.


I think you are correct, and I'll need to address the problem of ThunkSections empty on the first pass but not used. I think https://reviews.llvm.org/D34692 would be the better place to do so as this is where multiple places are introduced. Would it be possible to make the comment in https://reviews.llvm.org/D34692, or I can copy it across?

I deliberately didn't try and remove Thunks that are redundant, I'm expecting redundant thunks to only happen when there is a Thunk that has been knocked out of range of all of its callers and I think this will be rare enough to not warrant the extra complexity.


https://reviews.llvm.org/D34689