[Bug target/61044] Computed goto on AVR fails to use word-addressing
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61044 --- Comment #9 from Georg-Johann Lay --- (In reply to Senthil Kumar Selvaraj from comment #3) > The primary reason I added the diff relocs was to prevent linker relaxation > messing up DWARF line number information - as you know, relaxation can > shorten instruction sequences, and the addresses in DWARF then go out of > sync. > > I guess I must add some user documentation about this, but ideally, this is > supposed to be transparent to the user - just passing -mrelax to the > compiler should work. > > I turned diff reloc generation on only if -mlink-relax is passed because > this is what other ports (xtensa) do, and I wasn't sure of the consequences > of resolving every subtraction expression at link time. Resolving label differences at assemble time serves a faster linking process, but that argument does not apply to avr: We don't have magabytes of code that have to be fixed at load time by a dynamic linker. And you don't know at assemble time how the linker is called. One example is debugging through code that comes from a library and has been linked against the application. It's not very common but possible and yet another plus (besides simplicity with less options and less GCC/Binutils dependency) for always emitting label differences as relocs.
[Bug target/61044] Computed goto on AVR fails to use word-addressing
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61044 Georg-Johann Lay changed: What|Removed |Added Priority|P3 |P4 Status|UNCONFIRMED |RESOLVED Resolution|--- |INVALID Target Milestone|--- |4.9.1 --- Comment #8 from Georg-Johann Lay --- Closed as INVALID after adding a note to the manual that label differences are not supported for AVR.
[Bug target/61044] Computed goto on AVR fails to use word-addressing
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61044 --- Comment #7 from Georg-Johann Lay --- Author: gjl Date: Wed May 28 08:50:18 2014 New Revision: 211002 URL: http://gcc.gnu.org/viewcvs?rev=211002&root=gcc&view=rev Log: PR target/61044 * doc/extend.texi (Local Labels): Note that label differences are not supported for AVR. Modified: branches/gcc-4_7-branch/gcc/ChangeLog branches/gcc-4_7-branch/gcc/doc/extend.texi
[Bug target/61044] Computed goto on AVR fails to use word-addressing
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61044 --- Comment #6 from Georg-Johann Lay --- Author: gjl Date: Wed May 28 08:48:03 2014 New Revision: 211001 URL: http://gcc.gnu.org/viewcvs?rev=211001&root=gcc&view=rev Log: PR target/61044 * doc/extend.texi (Local Labels): Note that label differences are not supported for AVR. Modified: branches/gcc-4_8-branch/gcc/ChangeLog branches/gcc-4_8-branch/gcc/doc/extend.texi
[Bug target/61044] Computed goto on AVR fails to use word-addressing
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61044 --- Comment #5 from Georg-Johann Lay --- Author: gjl Date: Wed May 28 08:44:23 2014 New Revision: 211000 URL: http://gcc.gnu.org/viewcvs?rev=211000&root=gcc&view=rev Log: PR target/61044 * doc/extend.texi (Local Labels): Note that label differences are not supported for AVR. Modified: branches/gcc-4_9-branch/gcc/ChangeLog branches/gcc-4_9-branch/gcc/doc/extend.texi
[Bug target/61044] Computed goto on AVR fails to use word-addressing
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61044 --- Comment #4 from Georg-Johann Lay --- Author: gjl Date: Wed May 28 08:42:25 2014 New Revision: 210999 URL: http://gcc.gnu.org/viewcvs?rev=210999&root=gcc&view=rev Log: PR target/61044 * doc/extend.texi (Local Labels): Note that label differences are not supported for AVR. Modified: trunk/gcc/ChangeLog trunk/gcc/doc/extend.texi
[Bug target/61044] Computed goto on AVR fails to use word-addressing
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61044 --- Comment #3 from Senthil Kumar Selvaraj --- Johann, The primary reason I added the diff relocs was to prevent linker relaxation messing up DWARF line number information - as you know, relaxation can shorten instruction sequences, and the addresses in DWARF then go out of sync. I guess I must add some user documentation about this, but ideally, this is supposed to be transparent to the user - just passing -mrelax to the compiler should work. I turned diff reloc generation on only if -mlink-relax is passed because this is what other ports (xtensa) do, and I wasn't sure of the consequences of resolving every subtraction expression at link time. I tried assembling .word pm(.L3-.L2) with -mlink-relax, but the assembler gave up with an "expression too complex" error. Like you said, I guess documenting that this is not supported is the way to go.
[Bug target/61044] Computed goto on AVR fails to use word-addressing
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61044 Georg-Johann Lay changed: What|Removed |Added CC||gjl at gcc dot gnu.org, ||senthil_kumar.selvaraj@atme ||l.com --- Comment #2 from Georg-Johann Lay --- I don't see any difference in generated code with that patch. Anyway, the question is /what/ the generated code should be? The label differences would be something like .word pm(.L3-.L2) because we must not generate stubs for the offset. pm() works under the assumption that gas does not resolve the difference and emits relocs for it, and that pm() accepts differences. Currently, gas will compute the difference and come up with somethink like pm(4) which it resolves to 4 and not as 2. Senthil, I searched for a user level documentation of -mlink-relax but with no avail. Is there a special reason for why there is no documentation for that new gas command line option? And why is this option needed in the first place? Why not always emitting relocs for differences of labels? It does not buy us anything if gas computes the difference ... except for annoyance like checking for -mlink-relax during GCC configury. What does not work is to add offsets to stub addresses, e.g. gs(.L2)+4 will point to the next stub (if any) and *not* to .L2 + 4. Thus, I don't think it's worth to fix this PR and go through all the hassle with/without linker relaxation and cater for all situations. Instead, we should mention in the manual that the second example for computed goto (with label differences) is not supported for AVR. Not to mention that alternative #1 will yield better code because there is not need to add offsets at run time.
[Bug target/61044] Computed goto on AVR fails to use word-addressing
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=61044 --- Comment #1 from Dimitar Dimitrov --- The unofficial PRU GCC port (https://github.com/dinuxbg/gnupru) has the same bug. I haven't tested the following patch on AVR, but a similar change seems to fix the issue for PRU: --- a/gcc/config/avr/predicates.md +++ b/gcc/config/avr/predicates.md @@ -122,7 +122,7 @@ - (match_code "code_label,label_ref,symbol_ref,plus,const") + (match_code "code_label,label_ref,symbol_ref,plus,minus,const") @@ -133,6 +133,7 @@ case SYMBOL_REF : return SYMBOL_REF_FUNCTION_P (op); case PLUS : +case MINUS :
