On 1/4/20 9:48 PM, Fangrui Song wrote:
-fpatchable-function-entry is used by Linux kernel arm64/parisc. The feature works for x86, but does not leverage multi-byte NOP.% cat a.c int foo() { return 0; } % gcc -fpatchable-function-entry=5,0 -c a.c % objdump -d a.o ... 0000000000000000 <foo>: 0: 90 nop 1: 90 nop 2: 90 nop 3: 90 nop 4: 90 nop 5: 55 push %rbp 6: 48 89 e5 mov %rsp,%rbp 9: b8 00 00 00 00 mov $0x0,%eax e: 5d pop %rbp f: c3 retq (I am not sure who should be notified. I CCed x86-64-port maintainer (Jan Hubicka <hubi...@ucw.cz>), as listed on gcc/MAINTAINERS.)
Hello. You are right, we do not leverage multi-byte NOPs. Note that the support depends on a CPU model (-march) and the similar code is quite complex in binutils: https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;a=blob;f=gas/config/tc-i386.c;h=d0b8f2624a1885d83d2595474bfd78ae844f48f2;hb=HEAD#l1441 I'm not sure how worthy would it be to implement that? Thanks, Martin
