On 30/09/2024 4:15 pm, Jan Beulich wrote: > To fold two 32-bit outputs from the asm()-s into a single 64-bit value > the compiler needs to emit a zero-extension insn for the low half. Both > RDMSR and RDTSC clear the upper halves of their output registers anyway, > though. So despite that zero-extending insn (a simple MOV) being cheap, > we can do better: Without one, by declaring the local variables as 64- > bit ones. > > Signed-off-by: Jan Beulich <[email protected]>
Hmm. This is generally better, but not universally so. xen$ ../scripts/bloat-o-meter xen-syms-before xen-syms-after add/remove: 0/0 grow/shrink: 2/29 up/down: 24/-193 (-169) Function old new delta pci_cfg_ok 335 351 +16 arch_ioreq_server_get_type_addr 251 259 +8 udelay 81 79 -2 tsc_get_info 258 256 -2 trace 1320 1318 -2 time_latch_stamps 76 74 -2 set_legacy_ssbd.isra 146 144 -2 read_hyperv_timer 81 79 -2 read_counter 47 45 -2 read_clocks_slave 135 133 -2 mcheck_mca_logout 2059 2057 -2 hwp_write_request 105 103 -2 get_s_time_fixed 91 89 -2 get_s_time 77 75 -2 cpu_init 251 249 -2 amd_mcheck_init 754 752 -2 _svm_cpu_up 467 465 -2 tsc_check_writability 187 184 -3 _probe_mask_msr 106 99 -7 time_calibration_tsc_rendezvous 541 533 -8 time_calibration_std_rendezvous 197 189 -8 probe_cpuid_faulting 180 172 -8 hvm_save 343 335 -8 amd_check_erratum_1474 140 132 -8 calibrate_tsc 382 372 -10 wait_for_nmis 134 119 -15 read_msr 1220 1204 -16 guest_rdmsr 1869 1853 -16 check_tsc_warp.constprop 201 185 -16 hwp_init_msrs 440 420 -20 amd_log_freq.cold 519 499 -20 Total: Before=3895379, After=3895210, chg -0.00% arch_ioreq_server_get_type_addr goes from: and $0x40,%dh je ... to shl $0x20,%rdx or %rax,%rdx bt $0x2e,%rdx jae ... and pci_cfg_ok gets a "movabs $0x400000000000,%rcx" and 64bit test, where previously it used "and $0x40,%dh" So, the use of 64bit variables as opposed to 32bit does seem to break GCC's ability to substitute %dh. Oh well - it's minor either way. Reviewed-by: Andrew Cooper <[email protected]>
