http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57333
Bug ID: 57333
Summary: Wrong detection of LZCNT instruction, -mno-lzcnt has
no effect
Product: gcc
Version: 4.7.3
Status: UNCONFIRMED
Severity: major
Priority: P3
Component: target
Assignee: unassigned at gcc dot gnu.org
Reporter: bernhard.hartleb at gmail dot com
Sorry for describing two issues here, but I cant say how they are related:
1st: When using -march=native, gcc tries to use the LZCNT instruction. However
the CPUID of my processor does not show that this instruction is supported.
2nd: Trying to work around this, I noticed that -mno-lzcnt has no effect. When
using -march=amdfam10 (or native in my case) LZCNT will always be used and can
NOT be disabled.
Version:
gcc (Gentoo 4.7.3 p1.0, pie-0.5.5) 4.7.3
Relevant parts of /proc/cpuinfo:
vendor_id : AuthenticAMD
cpu family : 16
model : 4
model name : AMD Phenom(tm) II X4 810 Processor
stepping : 2
microcode : 0x10000c6
flags : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov
pat pse36 clflush mmx fxsr sse sse2 ht syscall nx mmxext fxsr_opt pdpe1gb
rdtscp lm 3dnowext 3dnow constant_tsc rep_good nopl nonstop_tsc extd_apicid pni
monitor cx16 popcnt lahf_lm cmp_legacy svm extapic cr8_legacy abm sse4a
misalignsse 3dnowprefetch osvw ibs skinit wdt hw_pstate npt lbrv svm_lock
nrip_save
(POPCNT is supported but LZCNT is not)
Testcase for the error:
$ echo | gcc -dM -E - -march=amdfam10 -mno-lzcnt | grep LZCNT
#define __LZCNT__ 1
Expected behaviour (like for POPCNT)
$ echo | gcc -dM -E - -march=amdfam10 -mno-popcnt | grep POP
(No output)
To sum it up: LZCNT is not detected correctly and can not be controlled.
