[Bug target/82430] New: [4.9 Regression] Suboptimal code generated because of unnecessary "pxor xmm0, xmm0"

["antoshkka at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82430

            Bug ID: 82430
           Summary: [4.9 Regression] Suboptimal code generated because of
                    unnecessary "pxor xmm0, xmm0"
           Product: gcc
           Version: 8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: antoshkka at gmail dot com
  Target Milestone: ---

Following code snippets

unsigned my_mul(unsigned a) {
    return a * 1.5;
}

unsigned my_div(unsigned a) {
    return a / 1.5;
}


Generate assemblies that have "pxor xmm0, xmm0" as a first instruction:

my_mul(unsigned int):
  pxor xmm0, xmm0   <=== This is not necessary
  mov edi, edi
  cvtsi2sdq xmm0, rdi
  mulsd xmm0, QWORD PTR .LC0[rip]
  cvttsd2si rax, xmm0
  ret

my_div(unsigned int):
  pxor xmm0, xmm0   <=== This is not necessary
  mov edi, edi
  cvtsi2sdq xmm0, rdi
  divsd xmm0, QWORD PTR .LC0[rip]
  cvttsd2si rax, xmm0
  ret


The regression was introduced with GCC 4.9. GCC-4.8 and previous versions were
generating assembly without pxor:

my_mul(unsigned int):
  mov edi, edi
  cvtsi2sd xmm0, rdi
  mulsd xmm0, QWORD PTR .LC0[rip]
  cvttsd2si rax, xmm0
  ret

my_div(unsigned int):
  mov edi, edi
  cvtsi2sd xmm0, rdi
  divsd xmm0, QWORD PTR .LC0[rip]
  cvttsd2si rax, xmm0
  ret