[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [PATCH] x86/altcall: always use a temporary parameter stashing variable
On Wed, Feb 28, 2024 at 03:28:25PM +0100, Jan Beulich wrote: > On 28.02.2024 14:59, Roger Pau Monne wrote: > > The usage in ALT_CALL_ARG() on clang of: > > > > register union { > > typeof(arg) e; > > const unsigned long r; > > } ... > > > > When `arg` is the first argument to alternative_{,v}call() and > > const_vlapic_vcpu() is used results in clang 3.5.0 complaining with: > > > > arch/x86/hvm/vlapic.c:141:47: error: non-const static data member must be > > initialized out of line > > alternative_call(hvm_funcs.test_pir, const_vlapic_vcpu(vlapic), > > vec) ) > > > > Workaround this by pulling `arg1` into a local variable, like it's done for > > further arguments (arg2, arg3...) > > > > Originally arg1 wasn't pulled into a variable because for the a1_ register > > local variable the possible clobbering as a result of operators on other > > variables don't matter: > > > > https://gcc.gnu.org/onlinedocs/gcc/Local-Register-Variables.html#Local-Register-Variables > > > > Note clang version 3.8.1 seems to already be fixed and don't require the > > workaround, but since it's harmless do it uniformly everywhere. > > > > Reported-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx> > > Fixes: 2ce562b2a413 ('x86/altcall: use a union as register type for > > function parameters on clang') > > Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx> > > I'm okay with this change, but since you don't mention anything in this > regard: Did you look at whether / how generated code (with gcc) changes? So the specific example of vlapic_test_irq() shows no changes to the generated code. bloat-o-meter shows a 0 delta: add/remove: 0/0 grow/shrink: 0/0 up/down: 0/0 (0) Function old new delta Total: Before=3570098, After=3570098, chg +0.00% I assume there can be some reordering of instructions, as we now force arg1 temporary variable (v1_) to be initialized ahead of the rest of the arguments. Thanks, Roger.
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