https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82328
Bug ID: 82328
Summary: x86 rdrand: flags not used directly when branching on
success/failure
Product: gcc
Version: 8.0
Status: UNCONFIRMED
Keywords: missed-optimization
Severity: normal
Priority: P3
Component: target
Assignee: unassigned at gcc dot gnu.org
Reporter: peter at cordes dot ca
Target Milestone: ---
Target: x86_64-*-*, i?86-*-*
#include <immintrin.h>
unsigned long long use_intrinsic(void) {
unsigned long long rand;
while(!_rdrand64_step(&rand)); // FIXME: limited retry in case RNG is
broken
return rand;
}
// https://godbolt.org/g/x7mUvj
gcc 8.0.0 20170926 -O3 -mrdrnd
movl $1, %edx
.L4:
rdrand %rax
movq %rax, -8(%rsp) # spill to memory, really?
cmovc %edx, %eax
testl %eax, %eax
je .L4
movq -8(%rsp), %rax
ret
Note that RDRAND (http://felixcloutier.com/x86/RDRAND.html) indicates failure
by clearing CF *and* putting 0 in the destination register. So this code is
correct (returning a valid RDRAND result even if it was zero), just much worse
than clang's:
.LBB1_1:
rdrandq %rax
jae .LBB1_1
retq
