I do not think that is possible. I do not think it is useful to have such operation. Why do you want it? If you have such hard requirement then you should just use std::mutex locked around a = b assignment.
UPDATE
I have tested the __transaction_atomic block with Cygwin64's GCC 4.8.1 and this very short source
extern int a, b;void foo (){ __transaction_atomic { a = b; }}results int oodles of instruction calling ITM library functions
_Z3foov:.LFB0: pushq %rdi # .seh_pushreg %rdi pushq %rsi # .seh_pushreg %rsi subq $200, %rsp #, .seh_stackalloc 200 movaps %xmm6, 32(%rsp) #, .seh_savexmm %xmm6, 32 movaps %xmm7, 48(%rsp) #, .seh_savexmm %xmm7, 48 movaps %xmm8, 64(%rsp) #, .seh_savexmm %xmm8, 64 movaps %xmm9, 80(%rsp) #, .seh_savexmm %xmm9, 80 movaps %xmm10, 96(%rsp) #, .seh_savexmm %xmm10, 96 movaps %xmm11, 112(%rsp) #, .seh_savexmm %xmm11, 112 movaps %xmm12, 128(%rsp) #, .seh_savexmm %xmm12, 128 movaps %xmm13, 144(%rsp) #, .seh_savexmm %xmm13, 144 movaps %xmm14, 160(%rsp) #, .seh_savexmm %xmm14, 160 movaps %xmm15, 176(%rsp) #, .seh_savexmm %xmm15, 176 .seh_endprologue movl $43, %edi #, xorl %eax, %eax # call _ITM_beginTransaction # testb $2, %al #, tm_state.4 je .L2 #, movq .refptr.b(%rip), %rax #, tmp67 movl (%rax), %edx # b, b movq .refptr.a(%rip), %rax #, tmp66 movl %edx, (%rax) # b, a movaps 32(%rsp), %xmm6 #, movaps 48(%rsp), %xmm7 #, movaps 64(%rsp), %xmm8 #, movaps 80(%rsp), %xmm9 #, movaps 96(%rsp), %xmm10 #, movaps 112(%rsp), %xmm11 #, movaps 128(%rsp), %xmm12 #, movaps 144(%rsp), %xmm13 #, movaps 160(%rsp), %xmm14 #, movaps 176(%rsp), %xmm15 #, addq $200, %rsp #, popq %rsi # popq %rdi # jmp _ITM_commitTransaction # .p2align 4,,10.L2: movq .refptr.b(%rip), %rcx #, call _ITM_RU4 # movq .refptr.a(%rip), %rcx #, movl %eax, %edx # D.2368, call _ITM_WU4 # call _ITM_commitTransaction # nop movaps 32(%rsp), %xmm6 #, movaps 48(%rsp), %xmm7 #, movaps 64(%rsp), %xmm8 #, movaps 80(%rsp), %xmm9 #, movaps 96(%rsp), %xmm10 #, movaps 112(%rsp), %xmm11 #, movaps 128(%rsp), %xmm12 #, movaps 144(%rsp), %xmm13 #, movaps 160(%rsp), %xmm14 #, movaps 176(%rsp), %xmm15 #, addq $200, %rsp #, popq %rsi # popq %rdi # ret .seh_endproc .ident "GCC: (GNU) 4.8.1" .def _ITM_beginTransaction; .scl 2; .type 32; .endef .def _ITM_commitTransaction; .scl 2; .type 32; .endef .def _ITM_RU4; .scl 2; .type 32; .endef .def _ITM_WU4; .scl 2; .type 32; .endef .section .rdata$.refptr.b, "dr" .globl .refptr.b .linkonce discard.refptr.b: .quad b .section .rdata$.refptr.a, "dr" .globl .refptr.a .linkonce discard.refptr.a: .quad aThis was with -O3 option.