http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58384
Bug ID: 58384 Summary: [4.9 regression] Runfail on spec2000/253.perlbmk if lto and pre-reload scheduler is used on x86 after r200133. Product: gcc Version: 4.9.0 Status: UNCONFIRMED Severity: normal Priority: P3 Component: rtl-optimization Assignee: unassigned at gcc dot gnu.org Reporter: ysrumyan at gmail dot com We also assume that arm can have the same problem at given benchmark if -flto is used to compile since pre-reload scheduler is used by default on it. The issue can be reproduced at attached test-case extracted from 253.perlbmk sources (function Perl_pp_entersub, file pp_hot.c). Before r200133 forward propagation (fwprop1 for rtl) performs substitution of stored value to subsequent load with the same base, but after this fix does not. Looking at assembly files for test-case we can see that -- before fix -- call realloc movq (%rbx), %r8 movq %rax, %r9 movq %rax, %rcx movq 8(%rsp), %rdx movq %rax, 24(%r8) movq %rax, (%r8) jmp .L4 i.e. return value of 'realloc' is copied to %r9 and %rcx which correspondent to pre temporaries. -- after fix -- (without pre-reload scheduler) call realloc movq (%rbx), %rcx movq (%rbx), %r8 movq 8(%rsp), %rdx movq %rax, 24(%rcx) movq %rax, (%rcx) movq (%r8), %rcx <--- redundant load movq %rcx, %rax jmp .L4 i.w. we can see redundant load from %r8. But if we turn on pre-reload scheduler, this load will be unlegal hoisted cross interleaved store (we got RT error on 253.perlbmk) and we can see it in assembly: call realloc movq (%rbx), %r8 movq 8(%rsp), %rdx movq (%r8), %rcx <--- non-legal code motion movq %rax, 24(%r8) movq %rax, (%r8) movq %rcx, %rax jmp .L4 To reproduce compile attached test-case with options -Ofast -march=corei7 -fschedule-insns --param sched-pressure-algorithm=1 -fsched-pressure