[PATCH] D42095: [WebAssembly] Symbol changes #3: Make Clang object file tables match LLD output. NFC.
Sam Clegg via Phabricator via llvm-commits
llvm-commits at lists.llvm.org
Tue Jan 23 15:02:16 PST 2018
sbc100 added inline comments.
================
Comment at: test/MC/WebAssembly/weak-alias.ll:197
+; CHECK-NEXT: Value: 16
+; CHECK-NEXT: Content: '01000000'
+; CHECK-NEXT: - Type: CUSTOM
----------------
ncw wrote:
> sbc100 wrote:
> > sbc100 wrote:
> > > ncw wrote:
> > > > sbc100 wrote:
> > > > > How as this change generated a new data segment? I guess maybe it was zero before so now it was elided?
> > > > >
> > > > > It looks to me like we really want to two different function addresses here for the address of the function itself and the address of the weak alias. Its not until link time that these two things become the same thing. Maybe it doesn't really matter as long as the linker can distinguish them, but it makes sense to be that there would be two different function addresses in this object file.
> > > > Ah, the test data before was skipping the section - see the "CHECK" in the original file, rather than "CHECK-NEXT". I thought it was useful to expand the test to assert on the section that was being skipped; it was odd that previously it was deliberately not examining the final segment.
> > > >
> > > > "It's not until link time that they become the same thing" -> but the table in the object file is linked. The "provisional" values that are used for relocations in the object file are a preview of the values that would get written out. I think it makes sense to use the same rules for writing out provisional values as we do for writing out the actual values in LLD. Hence we don't put the exact same function twice in the table - there's really no point. Seeing a function appear multiple times in the table doesn't make the output any easier to read.
> > > But for intermediate output such as this (or the lld -r) we are still waiting for the a potential strong version of the symbol, so the weak alias and the function in points too are still district (both here and in lld -r).
> > >
> > Hmm.. maybe i'll expand this test expectation now as a separate change to make this change more explicit.
> Yes - LLD might well resolve the indexes separately. But it doesn't seem to me that there's any added clarity in making the object file different to the linked output here (ie we're just outputting the same table that you'd get if you run LLD to link the single object file).
>
> The output you're requesting is going to look like this. It doesn't seem to me to assist in examining the object file to put the same function twice in the table:
>
> ```
> (table 3 anyfunc)
> (elem (i32.const 1) $func $func)
> (memory $0 1)
> (export "memory" (memory $0))
> (export "func" (func $func))
> (export "funcAlias" (func $func)) // Weak alias for func
> (func $func (type blah)
> )
> (func $anotherFunc (type blah)
> // A call to the alias and a call to the non-alias...
> // But there's no way to tell which is which! The alias is weakly-
> // defined, so it's not an import
> (call_indirect 1)
> (call_indirect 2)
> )
> ```
>
> Of course the other alternative is just to output a totally empty table in object files!
I'm taking a look at the `lld -r` output now. I'm sure we can agree that the clang output should match that of `lld -r` more that it matches just `lld` right?
It does looks like `lld -r` currently doesn't do the useful thing I was expecting.
The output if `wasm-objdump -x -d -r` a lot more useful to me that anything in llvm right now (until we can disassembly wasm files, which is coming soon). Hopefully we will have better tools that objtoyaml to use in our test cases at some point.
In this case I modified `lld/test/wasm/weak-alias.ll` to use `--relocatable` and I got the following:
```
0000cd <call_alias_ptr>:
0000d1: 23 80 80 80 80 00 | get_global 0
0000d2: R_WEBASSEMBLY_GLOBAL_INDEX_LEB 0
0000d7: 41 10 | i32.const 16
0000d9: 6b | i32.sub
0000da: 22 00 | tee_local 0
0000dc: 24 80 80 80 80 00 | set_global 0
0000dd: R_WEBASSEMBLY_GLOBAL_INDEX_LEB 0
0000e2: 20 00 | get_local 0
0000e4: 41 80 80 80 80 00 | i32.const 0
0000e5: R_WEBASSEMBLY_TABLE_INDEX_SLEB 1
0000ea: 36 02 08 | i32.store 2 8
0000ed: 10 81 80 80 80 00 | call 1 <direct_fn>
0000ee: R_WEBASSEMBLY_FUNCTION_INDEX_LEB 1 <direct_fn>
0000f3: 21 01 | set_local 1
0000f5: 20 00 | get_local 0
0000f7: 41 10 | i32.const 16
0000f9: 6a | i32.add
0000fa: 24 80 80 80 80 00 | set_global 0
0000fb: R_WEBASSEMBLY_GLOBAL_INDEX_LEB 0
000100: 20 01 | get_local 1
000102: 0b | end
```
and
```
000103 <call_direct_ptr>:
000107: 23 80 80 80 80 00 | get_global 0
000108: R_WEBASSEMBLY_GLOBAL_INDEX_LEB 0
00010d: 41 10 | i32.const 16
00010f: 6b | i32.sub
000110: 22 00 | tee_local 0
000112: 24 80 80 80 80 00 | set_global 0
000113: R_WEBASSEMBLY_GLOBAL_INDEX_LEB 0
000118: 20 00 | get_local 0
00011a: 41 81 80 80 80 00 | i32.const 1
00011b: R_WEBASSEMBLY_TABLE_INDEX_SLEB 1
000120: 36 02 08 | i32.store 2 8
000123: 10 81 80 80 80 00 | call 1 <direct_fn>
000124: R_WEBASSEMBLY_FUNCTION_INDEX_LEB 1 <direct_fn>
000129: 21 01 | set_local 1
00012b: 20 00 | get_local 0
00012d: 41 10 | i32.const 16
00012f: 6a | i32.add
000130: 24 80 80 80 80 00 | set_global 0
000131: R_WEBASSEMBLY_GLOBAL_INDEX_LEB 0
000136: 20 01 | get_local 1
000138: 0b | end
```
I was hoping that the two R_WEBASSEMBLY_FUNCTION_INDEX_LEB entries where would be different, as I think they should be. So I think that might be bug in the `--relocatable` output generation.
Repository:
rL LLVM
https://reviews.llvm.org/D42095
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