It was added in rS218893 as part of a Clang update. The new Clang version added the x86 integrated assembler but did not support .codeNN directives. .codeNN support has now been available in Clang for quite some time and there is no need to disable the integrated assembler.
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I think this is a good idea, but the last time I tried this, I compared the resulting object files, and I did see a bunch of differences with the output of gas. Is that still the case? It may just be due to a slightly different interpretation of prefixes and the like, but it is important to make sure that everything still boots after this change. :)
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Yes important point - now I seem to recall we spoke about this in the past and you pointed this out but I can't find details. Perhaps it was on IRC. I have been running a similar patch in test trees for quite some time, but will do a more detailed comparison of the differences.
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so long as we still boot, I'm cool. I'd be cooler if at least one of these .S files had a side by side comparison with the .o generated.... they should be substantially the same if not identical.
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I did not test or compare all of the resulting objects initially, and when I did later on I found a few issues of concern. I may try incrementally removing these, comparing object files and either finding them identical or at least having understandable differences.
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@dim compared in-tree gas and Clang 5.0 IAS and found a few differences in boot0:
gas: 81 c7 06 00 add $0x6,%di ias: 83 c7 06 add $0x6,%di
gas: 05 be 07 add $0x7be,%ax ias: 81 c0 be 07 add $0x7be,%ax
(and related offset changes)
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side by side comparison with the .o generated
Looking at boot1.o differences there are lots of header, debug, etc. changes, and changes due to differing offsets. The real change is different instruction size for the test instruction at the beginning of read
read: testb $FL_PACKET,%cs:MEM_REL+flags-start # LBA support enabled?
GNU as produces
0000:012e 2e f6 06 b0 08 80 test byte cs:[0x8b0], 0x80 ; [0x8b0:1]=255 ; 128
While Clang IAS produces:
0000:012e 2e 67 f6 05 b3 08 00 00 80 test byte cs:[0x8b3], 0x80 ; [0x8b3:1]=255 ; 128
This is fine for boot2, other than using three bytes more than necessary.
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If this is the only change, we can commit. boot1.o isn't in danger of running out of it's 512byte limited space, so we don't have to do special things.
None of the others have space concerns at all worth noting. Do we know if the differences are similar there as well?
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Do we know if the differences are similar there as well?
I believe so - the same issue is present in the others, but I have not yet confirmed that it's the only source of differences.
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cdboot differences are a large number of addr32 prefixes and using nop nop for alignment rather than xchg %eax,%eax, so it is ok too
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rebase after committing boot2 and cdboot changes
need to confirm that pxeboot and gptzfsboot are also only instruction size differences before committing those
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all other boot components have been addressed; only gptzfsboot needs to be investigated now
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As it turns out gptzfsboot is identical, whether GAS or IAS is used to assemble gptldr.S:
SHA256 (gas/gptzfsboot) = bc732937deaf7fd5c2d2bcaa1ec426d6f5e2c7bbf50922930f49a397cdec9820
SHA256 (ias/gptzfsboot) = bc732937deaf7fd5c2d2bcaa1ec426d6f5e2c7bbf50922930f49a397cdec9820
gptldr.o .text is identical, differences between gas and IAS are
- ELF headers
- DWARF debug info (Clang IAS adds it)
- some minor differences in symbols and relocations
- string table differences due to additional sections etc.