Example #1
0
// zerorange clears the stack in the given range.
func zerorange(p *obj.Prog, frame int64, lo int64, hi int64) *obj.Prog {
	cnt := hi - lo
	if cnt == 0 {
		return p
	}

	// Adjust the frame to account for LR.
	frame += gc.Ctxt.FixedFrameSize()
	offset := frame + lo
	reg := int16(s390x.REGSP)

	// If the offset cannot fit in a 12-bit unsigned displacement then we
	// need to create a copy of the stack pointer that we can adjust.
	// We also need to do this if we are going to loop.
	if offset < 0 || offset > 4096-clearLoopCutoff || cnt > clearLoopCutoff {
		p = appendpp(p, s390x.AADD, obj.TYPE_CONST, 0, offset, obj.TYPE_REG, s390x.REGRT1, 0)
		p.Reg = int16(s390x.REGSP)
		reg = s390x.REGRT1
		offset = 0
	}

	// Generate a loop of large clears.
	if cnt > clearLoopCutoff {
		n := cnt - (cnt % 256)
		end := int16(s390x.REGRT2)
		p = appendpp(p, s390x.AADD, obj.TYPE_CONST, 0, offset+n, obj.TYPE_REG, end, 0)
		p.Reg = reg
		p = appendpp(p, s390x.AXC, obj.TYPE_MEM, reg, offset, obj.TYPE_MEM, reg, offset)
		p.From3 = new(obj.Addr)
		p.From3.Type = obj.TYPE_CONST
		p.From3.Offset = 256
		pl := p
		p = appendpp(p, s390x.AADD, obj.TYPE_CONST, 0, 256, obj.TYPE_REG, reg, 0)
		p = appendpp(p, s390x.ACMP, obj.TYPE_REG, reg, 0, obj.TYPE_REG, end, 0)
		p = appendpp(p, s390x.ABNE, obj.TYPE_NONE, 0, 0, obj.TYPE_BRANCH, 0, 0)
		gc.Patch(p, pl)

		cnt -= n
	}

	// Generate remaining clear instructions without a loop.
	for cnt > 0 {
		n := cnt

		// Can clear at most 256 bytes per instruction.
		if n > 256 {
			n = 256
		}

		switch n {
		// Handle very small clears with move instructions.
		case 8, 4, 2, 1:
			ins := s390x.AMOVB
			switch n {
			case 8:
				ins = s390x.AMOVD
			case 4:
				ins = s390x.AMOVW
			case 2:
				ins = s390x.AMOVH
			}
			p = appendpp(p, ins, obj.TYPE_CONST, 0, 0, obj.TYPE_MEM, reg, offset)

		// Handle clears that would require multiple move instructions with XC.
		default:
			p = appendpp(p, s390x.AXC, obj.TYPE_MEM, reg, offset, obj.TYPE_MEM, reg, offset)
			p.From3 = new(obj.Addr)
			p.From3.Type = obj.TYPE_CONST
			p.From3.Offset = n
		}

		cnt -= n
		offset += n
	}

	return p
}