GCC at least is generating horrible code for them, so do the bit twiddling ourselves. This is not premature optimization, because it might be the seed for a QBitField class, as suggested by Andrei Alexandrescu on GN2013. The assembler code of default and copy ctors is unchanged between the old and new versions. Accessors such as redSize() are virtually identical (with an and and a mov exchanging place). The interesting thing is what happend in operator== and the set-all-fields ctor. These are the disassemblies of functions wrapping op== and the ctor, resp.: bool pf_comparison(QPixelFormat, QPixelFormat): .cfi_startproc | .cfi_startproc movl %esi, %edx | cmpq %rsi, %rdi xorl %eax, %eax | sete %al xorl %edi, %edx | ret andl $15, %edx | .cfi_endproc jne .L53 | movl %esi, %edx | xorl %edi, %edx | testw $1008, %dx | jne .L53 | movl %esi, %edx | pushq %rbx | .cfi_def_cfa_offset 16 | .cfi_offset 3, -16 | movl %edi, %ebx | movzbl %dh, %ecx | movzbl %bh, %edx | xorl %ecx, %edx | andl $252, %edx | jne .L40 | movq %rdi, %rcx | movq %rsi, %rdx | shrq $16, %rcx | shrq $16, %rdx | movl %ecx, %ebx | xorl %edx, %ebx | movl %ebx, %r8d | andl $63, %r8d | jne .L40 | xorl %ecx, %edx | testw $4032, %dx | jne .L40 | movq %rsi, %rcx | movabsq $16911433728, %rdx | xorq %rdi, %rcx | testq %rdx, %rcx | jne .L40 | movq %rdi, %rdx | movq %rsi, %rcx | shrq $32, %rdx | shrq $32, %rcx | movl %edx, %ebx | xorl %ecx, %ebx | movl %ebx, %r8d | andl $252, %r8d | jne .L40 | movq %rsi, %r9 | movq %rdi, %r8 | shrq $40, %r9 | shrq $40, %r8 | xorl %r9d, %r8d | andl $127, %r8d | jne .L40 | xorl %ecx, %edx | andl $98304, %edx | jne .L40 | movq %rsi, %rcx | movq %rdi, %rdx | shrq $48, %rcx | shrq $48, %rdx | xorl %ecx, %edx | andl $126, %edx | jne .L40 | shrq $48, %rdi | movq %rcx, %rax | xorl %edi, %eax | testw $-128, %ax | sete %al | .p2align 4,,10 | .p2align 3 | .L40: | popq %rbx | .cfi_restore 3 | .cfi_def_cfa_offset 8 | .L53: | rep | ret | .cfi_endproc | That one is pretty obvious. Hint: the right one is the new version. QPixelFormat pf_unwieldy_ctor(QPixelFormat::ColorModel, ...) .cfi_startproc | .cfi_startproc movq %rbp, -32(%rsp) | movq %rbx, -40(%rsp) movq %r12, -24(%rsp) | .cfi_offset 3, -48 andl $15, %edi | movzbl 8(%rsp), %ebx movq %r13, -16(%rsp) | andl $63, %esi movq %r14, -8(%rsp) | movq %rbp, -32(%rsp) andl $63, %esi | movq %r12, -24(%rsp) movl 48(%rsp), %r11d | andl $63, %edx movzbl 8(%rsp), %r10d | .cfi_offset 6, -40 andl $63, %edx | .cfi_offset 12, -32 .cfi_offset 6, -40 | movl 16(%rsp), %ebp .cfi_offset 12, -32 | movl 32(%rsp), %r11d .cfi_offset 13, -24 | andl $63, %ecx .cfi_offset 14, -16 | movl 40(%rsp), %r10d movzbl 16(%rsp), %r14d | movq %r13, -16(%rsp) movzbl 24(%rsp), %r13d | andl $63, %r8d andl $63, %ecx | andl $63, %ebx movzbl 32(%rsp), %r12d | .cfi_offset 13, -24 movzbl 40(%rsp), %ebp | movl 48(%rsp), %r13d andl $63, %r8d | andl $63, %r9d movl %r11d, %eax | movq %rbx, %r12 movq %rbx, -40(%rsp) | movl 24(%rsp), %ebx .cfi_offset 3, -48 | andl $1, %ebp andl $63, %r9d | andl $1, %r11d andl $3, %eax | andl $15, %r10d andl $63, %r10d | movq %rdi, %rax xorl %ebx, %ebx | andl $15, %eax andl $1, %r14d | salq $4, %rsi andl $1, %r13d | salq $10, %rdx andl $1, %r12d | andl $1, %ebx andl $15, %ebp | salq $16, %rcx cmpl $2, %r11d | salq $22, %r8 cmovne %eax, %ebx | salq $28, %r9 andl $63, %esi | salq $34, %r12 movq %rdi, %rax | salq $40, %rbp salq $4, %rsi | salq $41, %rbx andl $15, %eax | salq $42, %r11 andl $63, %edx | salq $43, %r10 salq $10, %rdx | cmpl $2, %r13d orq %rsi, %rax | movq %r14, -8(%rsp) andl $63, %ecx | .cfi_offset 14, -16 salq $16, %rcx | movzbl 56(%rsp), %r14d orq %rdx, %rax | je .L45 andl $63, %r8d | andl $3, %r13d orq %rcx, %rax | salq $47, %r13 salq $22, %r8 |.L44: andl $63, %r9d | orq %rsi, %rax salq $28, %r9 | orq %rdx, %rax orq %r8, %rax | movq %r14, %rdx andl $63, %r10d | movq -8(%rsp), %r14 movq %r14, %rcx | orq %rcx, %rax salq $34, %r10 | andl $63, %edx orq %r9, %rax | orq %r8, %rax andl $1, %ecx | salq $49, %rdx movq %r13, %rdx | orq %r9, %rax orq %r10, %rax | orq %r12, %rax salq $40, %rcx | movq -24(%rsp), %r12 andl $1, %edx | orq %rbp, %rax movq -16(%rsp), %r13 | movq -32(%rsp), %rbp salq $41, %rdx | orq %rbx, %rax orq %rcx, %rax | movq -40(%rsp), %rbx movq %r12, %rcx | orq %r11, %rax orq %rdx, %rax | orq %r10, %rax andl $1, %ecx | orq %rdx, %rax movq %rbp, %rdx | orq %r13, %rax salq $42, %rcx | movq -16(%rsp), %r13 andl $15, %edx | ret movq -32(%rsp), %rbp | .p2align 4,,10 salq $43, %rdx | .p2align 3 orq %rcx, %rax |.L45: movq %rbx, %rcx | xorl %r13d, %r13d orq %rdx, %rax | jmp .L44 movzbl 56(%rsp), %edx | .cfi_endproc andl $3, %ecx | salq $47, %rcx | movq -40(%rsp), %rbx | movq -24(%rsp), %r12 | movq -8(%rsp), %r14 | andl $63, %edx | orq %rcx, %rax | salq $49, %rdx | orq %rdx, %rax | ret | .cfi_endproc | Without bothering to understand the details, they look pretty similar, with the new version being slightly shorter. But that may not mean anything. Change-Id: I31e84c9109ccd0c7282351b2e2802407a9b360b4 Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>