https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101891
Bug ID: 101891
Summary: Adjust -fzero-call-used-regs to always use XOR
Product: gcc
Version: unknown
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: middle-end
Assignee: unassigned at gcc dot gnu.org
Reporter: kees at outflux dot net
Target Milestone: ---
Currently -fzero-call-used-regs will use a pattern of:
XOR regA,regA
MOV regA,regB
MOV regA,regC
...
RET
However, this introduces both a register ordering dependency (e.g. the CPU
cannot clear regB without clearing regA first), and while greatly reduces
available ROP gadgets, it does technically leave a set of "MOV" ROP gadgets at
the end of functions (e.g. "MOV regA,regC; RET").
Please switch to always using XOR:
XOR regA,regA
XOR regB,regB
XOR regC,regC
...
RET
