On Thu, Feb 7, 2019 at 4:17 AM Borislav Petkov <b...@alien8.de> wrote: > > On Thu, Jan 31, 2019 at 11:24:09AM -0800, Thomas Garnier wrote: > > Replace the %c constraint with %P. The %c is incompatible with PIE > > because it implies an immediate value whereas %P reference a symbol. > > How so? > > AFAIK, %c requires a constant operand and if %P is used to print a > constant, it simply drops syntax-specific prefixes and does a bare > constant. > > I guess that here > > https://gcc.gnu.org/onlinedocs/gcc/Extended-Asm.html#x86Operandmodifiers > > is not entirely correct as it should not say "If used for a constant" > for %P but say "symbol or constant". > > But before/after asm doesn't show any difference. So what gives?
I assume that's an optimisation done by gcc later. The P modifier in the documentation does state that it is used to generate PIC code. > > before: > # 39 "./arch/x86/include/asm/jump_label.h" 1 > 1: > .byte 0xe9 > .long .L241 - 2f # > 2: > .pushsection __jump_table, "aw" > .balign 8 > .long 1b - ., .L241 - . # > .quad __use_tsc + 1 - . #, > .popsection > > after: > # 39 "./arch/x86/include/asm/jump_label.h" 1 > 1: > .byte 0xe9 > .long .L241 - 2f # > 2: > .pushsection __jump_table, "aw" > .balign 8 > .long 1b - ., .L241 - . # > .quad __use_tsc+1 - . # > .popsection > > -- > Regards/Gruss, > Boris. > > Good mailing practices for 400: avoid top-posting and trim the reply.
