Differential D136436
[Clang][LoongArch] Add GPR alias handling without `$` prefix Authored by SixWeining on Oct 21 2022, 2:49 AM.
Details Currenlty there is a mismatch between LoongArch gcc and clang about Take $r4 for example. With this patch, clang accepts $r4, r4,
Diff Detail
Event TimelineComment Actions Commit message is unclear. You added non-prefixed aliases for all registers, yet only $a0 is mentioned. I'd like to see what improvement you could come up with before amending it myself, to hopefully make you better at writing those "small compositions".
Comment Actions $ cat prefix_regalias.c int main() { register unsigned long a0 asm("a0"); register unsigned long a1 asm("$a1"); register unsigned long a2 asm("r6"); register unsigned long a3 asm("$r7"); register float f0 asm("fa0"); register float f1 asm("$fa1"); register float f2 asm("f2"); register float f3 asm("$f3"); return 0; } clang before applying patch: $ clang prefix_regalias.c prefix_regalias.c:3:32: error: unknown register name 'a0' in asm register unsigned long a0 asm("a0"); ^ prefix_regalias.c:5:32: error: unknown register name 'r6' in asm register unsigned long a2 asm("r6"); ^ prefix_regalias.c:7:24: error: unknown register name 'fa0' in asm register float f0 asm("fa0"); ^ prefix_regalias.c:9:24: error: unknown register name 'f2' in asm register float f2 asm("f2"); ^ 4 errors generated. clang after applying patch: $ clang prefix_regalias.c gcc: $ gcc prefix_regalias.c prefix_regalias.c: In function ‘main’: prefix_regalias.c:7:24: error: invalid register name for ‘f0’ 7 | register float f0 asm("fa0"); | ^~ prefix_regalias.c:8:24: error: invalid register name for ‘f1’ 8 | register float f1 asm("$fa1"); | Comment Actions For the assembler part of this support, I think we need to coordinate with the GNU toolchain maintainers of LoongArch port (Chenghua, Zhensong and Lulu I think). Maybe raising an issue on the LoongArch documentation repo would help. For the GCC part, consistency is of course welcomed, and I think the Loongson maintainers or @xry111 could just submit the respective support. Comment Actions Well. To be honest, I'd like to keep only one form but not both. Seems other archs only support one form? If we support both, will it make people confused? Is mixed form allowed (fmadd.d fa0, $fa0, f0, $fcc0) ? Comment Actions Updated the description of all registers, thanks for pointing it out.
Comment Actions @tangyouling Considering that $ is well supported by both gcc and llvm, we should adandon this patch? Comment Actions I'm afraid no, the prefix-less form is AFAIK likely inspired by RISC-V assembly (which doesn't have any prefix for registers whatsoever), but the $-prefixed form is clearly a MIPS style, and frankly speaking we (the community) had no choice at the time of binutils upstreaming because of the insistence of Chenghua et al., and loads of code using the prefixed form already appearing. The important point is that there's already code making use of prefix-less register names in inline asm, so we must follow suit for keeping source compatibility across gcc and clang. While having just one form to support is indeed nice, that ship has sailed long ago, and I think the next best thing is to allow both styles but disallow mixing of styles in one insn, i.e. no things like add $a0, a1, $a2, if it's impossible to only allow prefix-less register names in Clang inline asm but not at LLVM level. Comment Actions I would prefer that the treatment of aliases should be completely consistent in gcc and llvm (otherwise it will cause some trouble for developers). Comment Actions Is it acceptable to add the non-prefix $? if not, an alternative fix for the build failure is to add the prefix $ in sanitizer_syscall_linux_loongarch64.inc Comment Actions People have different opinions. This may need long time to discuss and decide (maybe in a broader place). So currently it's better to modify sanitizer_syscall_linux_loongarch64.inc I think. Comment Actions I wrote sanitizer_syscall_linux_loongarch64.inc and tested it with GCC. GCC supports register variable definition like register u64 a0 asm("a0"); but does not support addi.d a0, a0, a1 (because in GCC such a line in inline assembly is passed to GNU as directly). Is it possible to "emulate" the behavior in Clang? Comment Actions I don't have any objection to add $ into sanitizer_syscall_linux_loongarch64.inc though, GCC supports both register u64 a0 asm("a0"); and register u64 a0 asm("$a0"); Comment Actions Ok, I will send a new Revision to fix sanitizer_syscall_linux_loongarch64.inc separately, but this Revision does not close and continue the discussion. Comment Actions Hi, any update on this? Given the $-less style has been long established at this time, and present in released GCC versions, it seems we indeed have to follow suit (and later make GCC accept ABI names for FPRs too). It will also make the ClangBuiltLinux work easier. Comment Actions I tend to adopt @xry111's suggestion that "emulating" GCC's behavior in Clang (not in LLVM):
And for FPRs, I suggest to keep current implementation until GCC support non-prefixed names. This means:
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Missing "s9".