![]() | |||||||||||||
Messages in this thread |
On Friday 05 May 2006 15:20, Alan Modra wrote: > On Fri, May 05, 2006 at 11:45:54AM +0300, Denis Vlasenko wrote: > > > > .section .smp_altinstr_replacement,"awx" > > .section .sched.text,"ax",@progbits > > call _spin_unlock # > > 661: > > 2: jle 2b # > > 662: > > .section .smp_altinstr_replacement,"awx" > > .fill 662b-661b,1,0x42 > > gas should give a better error message here, but really, gas shouldn't > be expected to assemble this. In essence, you have forward references > in that expression for the .fill length.. Yes, this should be an error, not warning. It produces miscompiled object modules. > Some background: Gas is a single pass assembler. It emits code and > data into "frags", buffers containing some fixed number of bytes and > possibly a variable length tail. The variable length part allows > various features, notably that of variable length instructions. Symbols > are defined relative to their frags. Until the frag addresses are > finalized, an expresion involving subtraction of two symbols in > different frags cannot be evaluated correctly. With the testcase above > you have exactly that situation. The x86 "jle" instruction can be two > sizes, either 6 bytes or 2 bytes depending on the offset needed, and gas > doesn't have the smarts to recognize that the "jle" above is just 2 > bytes. Instead, it assumes a variable size, putting the "jle" in its > own frag. This means that label "661" and "662" are in separate frags > with "661" at offset 5 in its frag, and "662" at offset 0. > > Since you define the ".smp_altinstr_replacement" section before the > ".sched.text section", gas tries to finalize ".smp_altinstr_replacement" > first. When it tries to calculate the fill size using > (<base addr "662" frag>+<offset "662">) > - (<base addr "661" frag>+<offset "661">) > the frag base addresses have not yet been set, and zero is used. ie. > gas tries to assemble ".fill -5,1,0x42". Thanks for the explanation. -- vda - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/ | ||||||||||||
| Last update: 2006-05-05 13:16 [from the cache] ©2003-2008 | |||||||||||||