C++ 从函数返回 struct 时可能出现 GCC 错误

我相信我在执行O‘Neill的PCG PRNG时发现了GCC的错误.(Initial code on Godbolt's Compiler Explorer)

在将oldstate乘以MULTIPLIER(结果存储在rdi中)后，GCC不会将该结果添加到INCREMENT，而是将INCREMENT移动到rdx，然后将其用作rand32_ret.state的返回值

最小可复制示例(Compiler Explorer):

#include <stdint.h>

struct retstruct {
    uint32_t a;
    uint64_t b;
};

struct retstruct fn(uint64_t input)
{
    struct retstruct ret;

    ret.a = 0;
    ret.b = input * 11111111111 + 111111111111;

    return ret;
}

生成的程序集(GCC 9.2，x86_64，-O3):

fn:
  movabs rdx, 11111111111     # multiplier constant (doesn't fit in imm32)
  xor eax, eax                # ret.a = 0
  imul rdi, rdx
  movabs rdx, 111111111111    # add constant; one more 1 than multiplier
     # missing   add rdx, rdi   # ret.b=... that we get with clang or older gcc
  ret
# returns RDX:RAX = constant 111111111111 : 0
# independent of input RDI, and not using the imul result it just computed

有趣的是，将 struct 修改为将uint64_t作为第一个成员produces correct code,changing both members to be uint64_t也是如此

x86-64 system V确实会在rdx:rax中返回小于16字节的 struct ，如果它们很容易复制的话.在这种情况下，第二个构件在RDX中，因为Rax的上半部分是用于对齐的填充物，或者当.a是较窄的类型时是.b.(sizeof(retstruct)是16；我们不使用__attribute__((packed))，因此它遵循alignof(Uint64_T)=8.)

Does this code contain any undefined behaviour that would allow GCC to emit the "incorrect" assembly?

如果没有，则应在https://gcc.gnu.org/bugzilla/天内报告