[llvm-bugs] [Bug 39844] New: Field ordering causes extra memcpy

via llvm-bugs llvm-bugs at lists.llvm.org
Thu Nov 29 15:14:09 PST 2018


            Bug ID: 39844
           Summary: Field ordering causes extra memcpy
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Scalar Optimizations
          Assignee: unassignedbugs at nondot.org
          Reporter: jmuizelaar at mozilla.com
                CC: llvm-bugs at lists.llvm.org

The following program:

#include <stdlib.h>

struct SV {
        size_t capacity;
        size_t disc;
        size_t data[40];

        static SV make() {
                SV ret;
                ret.capacity = 0;
                ret.disc = 0;
                return ret;

struct L {
        SV a;

template<class T>
struct Allocation {
    T *vec;
    void init(T s) {
        *vec = s;

void bar(Allocation<L> a, double g) {
        L s = { SV::make() };


bar(Allocation<L>, double): # @bar(Allocation<L>, double)
  sub rsp, 344
  xorps xmm0, xmm0
  movaps xmmword ptr [rsp], xmm0
  mov rsi, rsp
  mov edx, 336
  call memcpy
  add rsp, 344

Moving the capacity field from the beginning of the struct to the end gives the
more desirable:

bar(Allocation<L>, double): # @bar(Allocation<L>, double)
  mov qword ptr [rdi], 0
  mov qword ptr [rdi + 328], 0

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