pcc wrote: Sorry, let me try again with the concrete problem that I was seeing and trying to solve.
With this .s file: ``` .section .text.a,"ax",@progbits jmp foo .section .text.b,"ax",@progbits bar: ret .text jmp bar foo: ret ``` and with an lld patched with #138366: ``` $ ra/bin/clang -c jmp2.s $ ra/bin/ld.lld jmp2.o --branch-to-branch ld.lld: warning: cannot find entry symbol _start; not setting start address $ objdump -d a.out: file format elf64-x86-64 Disassembly of section .text: 0000000000201120 <foo-0x5>: 201120: e9 06 00 00 00 jmp 20112b <bar> 0000000000201125 <foo>: 201125: c3 ret 201126: e9 05 00 00 00 jmp 201130 <bar+0x5> 000000000020112b <bar>: 20112b: c3 ret ``` The instruction at 0x201126 is incorrect. This is because of the interpretation of `jmp foo`. Currently it is assembled as `jmp .text+5` i.e. PLT32 with symbol `.text` and addend 1. With the current implementation of branch-to-branch this is treated as a branch to `.text` that needs 1 added to the operand (similar to the relative vtable case). Consequently it finds the relocation for `jmp bar` at the beginning of the section, assumes that is the target and incorrectly rewrites it. After #138795 we have: ``` $ objdump -d a.out: file format elf64-x86-64 Disassembly of section .text: 0000000000201120 <foo-0x5>: 201120: e9 06 00 00 00 jmp 20112b <bar> 0000000000201125 <foo>: 201125: c3 ret 201126: e9 fa ff ff ff jmp 201125 <foo> 000000000020112b <bar>: 20112b: c3 ret ``` The alternative fix, which I think I'm now leaning towards, would be to change how the branch-to-branch optimization handles relocations to STT_SECTION symbols. A relocation pointing to the STT_SECTION for .text with addend 1 would be treated as a branch to `.text+5` and it would be invalid to assemble a relative vtable relocation that points to STT_SECTION. It would also be consistent with how we process relocations for string tail merging and ICF among other places, e.g. [here](https://github.com/llvm/llvm-project/blob/8602a655a8150753542b0237fcca16d9ee1cd981/lld/ELF/ICF.cpp#L304). The downside is that it adds another special case for STT_SECTION but I guess that's fine. https://github.com/llvm/llvm-project/pull/138795 _______________________________________________ llvm-branch-commits mailing list llvm-branch-commits@lists.llvm.org https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-branch-commits
