atomic reordering

a.cc

#include <atomic>

int a;
std::atomic<bool> a_meaningful { 0 };  // a is meaningful only if a_meaningful

int Read() {
  if (a_meaningful.load(std::memory_order_acquire)) {
    return a;
  }
  return -1;
}

int Init() {
  a = 42;
  a_meaningful.store(1, std::memory_order_release);
}

/*
	.file	"a.cc"
	.machine power8
	.abiversion 2
	.section	".text"
	.align 2
	.p2align 4,,15
	.globl _Z4Readv
	.type	_Z4Readv, @function
_Z4Readv:
.LFB342:
	.cfi_startproc
.LCF0:
0:	addis 2,12,.TOC.-.LCF0@ha
	addi 2,2,.TOC.-.LCF0@l
	.localentry	_Z4Readv,.-_Z4Readv
	addis 9,2,.LANCHOR0@toc@ha		# gpr load fusion, type char
	lbz 9,.LANCHOR0@toc@l(9)
	cmpw 7,9,9
	bne- 7,$+4
	isync
	rlwinm 9,9,0,0xff
	cmpdi 7,9,0
	beq 7,.L3
	addis 9,2,.LANCHOR0+4@toc@ha
	lwa 3,.LANCHOR0+4@toc@l(9)
	blr
	.p2align 4,,15
.L3:
	li 3,-1
	blr
	.long 0
	.byte 0,9,0,0,0,0,0,0
	.cfi_endproc
.LFE342:
	.size	_Z4Readv,.-_Z4Readv
	.align 2
	.p2align 4,,15
	.globl _Z4Initv
	.type	_Z4Initv, @function
_Z4Initv:
.LFB343:
	.cfi_startproc
.LCF1:
0:	addis 2,12,.TOC.-.LCF1@ha
	addi 2,2,.TOC.-.LCF1@l
	.localentry	_Z4Initv,.-_Z4Initv
	li 10,42
	addis 8,2,.LANCHOR0+4@toc@ha
	li 9,1
	stw 10,.LANCHOR0+4@toc@l(8)
	addis 10,2,.LANCHOR0@toc@ha
	lwsync
	stb 9,.LANCHOR0@toc@l(10)
	blr
	.long 0
	.byte 0,9,0,0,0,0,0,0
	.cfi_endproc
.LFE343:
	.size	_Z4Initv,.-_Z4Initv
	.globl a_meaningful
	.globl a
	.section	".bss"
	.align 2
	.set	.LANCHOR0,. + 0
	.type	a_meaningful, @object
	.size	a_meaningful, 1
a_meaningful:
	.zero	1
	.zero	3
	.type	a, @object
	.size	a, 4
a:
	.zero	4
	.ident	"GCC: (GNU) 7.0.1 20170302 (experimental)"
	.section	.note.GNU-stack,"",@progbits
*/