On 2020-01-04, 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.)
Archaeology is difficult with the mailing list archive... CC Torsten as the patch author https://gcc.gnu.org/ml/gcc-patches/2016-09/msg02257.html [PATCH v2] add -fprolog-pad=N option to c-family https://gcc.gnu.org/ml/gcc-patches/2017-07/msg00391.html [PATCH v12] add -fpatchable-function-entry=N,M option Another finding is that -fpatchable-function-entry=N[,M] is very similar to -mhotpatch=pre-halfwords,post-halfwords on S/390 https://gcc.gnu.org/onlinedocs/gcc/S_002f390-and-zSeries-Options.html If the hotpatch option is enabled, a “hot-patching” function prologue is generated for all functions in the compilation unit. The funtion label is prepended with the given number of two-byte NOP instructions (pre-halfwords, maximum 1000000). After the label, 2 * post-halfwords bytes are appended, using the largest NOP like instructions the architecture allows (maximum 1000000).
