Messages in this thread | | | Date | Tue, 24 Jan 2023 14:06:49 +0100 | From | Peter Zijlstra <> | Subject | Re: [PATCH 3/3] x86/static_call: Add support for Jcc tail-calls |
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On Mon, Jan 23, 2023 at 05:44:31PM -0500, Steven Rostedt wrote: > On Mon, 23 Jan 2023 21:59:18 +0100 > Peter Zijlstra <peterz@infradead.org> wrote: > > > Clang likes to create conditional tail calls like: > > > > 0000000000000350 <amd_pmu_add_event>: > > 350: 0f 1f 44 00 00 nopl 0x0(%rax,%rax,1) 351: R_X86_64_NONE __fentry__-0x4 > > 355: 48 83 bf 20 01 00 00 00 cmpq $0x0,0x120(%rdi) > > 35d: 0f 85 00 00 00 00 jne 363 <amd_pmu_add_event+0x13> 35f: R_X86_64_PLT32 __SCT__amd_pmu_branch_add-0x4 > > 363: e9 00 00 00 00 jmp 368 <amd_pmu_add_event+0x18> 364: R_X86_64_PLT32 __x86_return_thunk-0x4 > > > > Just to confirm, as it's not clear if this is the static call site or one > of the functions that is being called.
Ah, you've not looked at enough asm then? ;-) Yes this is the static call site, see the __SCT_ target (instruction at 0x35d).
> I'm guessing that this is an issue because clang optimizes the static call > site, right?
Specifically using Jcc (jne in this case) to tail-call the trampoline.
> > Teach the in-line static call text patching about this. > > > > Notably, since there is no conditional-ret, in that caes patch the Jcc > > "in that case"
typing so hard.. :-)
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