Try not to copy outgoing arguments #23657
Description
I see stuff like this in Servo's disassembly for arguments that are moved:
+0x1bcc movq -6256(%rbp), %rax
+0x1bd3 movq %rax, -3344(%rbp)
+0x1bda movaps -6272(%rbp), %xmm0
+0x1be1 movaps %xmm0, -3360(%rbp)
+0x1be8 movaps -6288(%rbp), %xmm0
+0x1bef movaps %xmm0, -3376(%rbp)
+0x1bf6 movaps -6304(%rbp), %xmm0
+0x1bfd movaps %xmm0, -3392(%rbp)
+0x1c04 movaps -6368(%rbp), %xmm0
+0x1c0b movaps -6352(%rbp), %xmm1
+0x1c12 movaps -6336(%rbp), %xmm2
+0x1c19 movaps -6320(%rbp), %xmm3
+0x1c20 movaps %xmm3, -3408(%rbp)
+0x1c27 movaps %xmm2, -3424(%rbp)
+0x1c2e movaps %xmm1, -3440(%rbp)
+0x1c35 movaps %xmm0, -3456(%rbp)
+0x1c3c xorps %xmm0, %xmm0
+0x1c3f movaps %xmm0, -6272(%rbp)
+0x1c46 movaps %xmm0, -6288(%rbp)
+0x1c4d movaps %xmm0, -6304(%rbp)
+0x1c54 movaps %xmm0, -6320(%rbp)
+0x1c5b movaps %xmm0, -6336(%rbp)
+0x1c62 movaps %xmm0, -6352(%rbp)
+0x1c69 movaps %xmm0, -6368(%rbp)
+0x1c70 movq $0, -6256(%rbp)
+0x1c7b leaq -7600(%rbp), %rdi
+0x1c82 leaq -3456(%rbp), %rsi
+0x1c89 callq "vec_deque::VecDeque$LT$T$GT$::push_front::h12917901159388456219"
There's generally no need to copy by-move arguments. They should not be by-value at the LLVM level but should instead be by-pointer. Essentially codegen should opportunistically treat by-value as a hypothetical &own for aggregates.