clang's integrated assembler doesn't permit setting -Wa,-mimplicit-it=always for ARM

[nickdesaulniers]

Bugzilla Link	49023
Resolution	FIXED
Resolved on	Feb 11, 2021 10:57
Version	trunk
OS	All
Blocks	#4440
CC	@arndb,@jcai19,@DavidSpickett,@kbeyls,@lalozano,@zygoloid,@stephenhines

Extended Description

While it seems we can use the internal flag -mllvm --arm-implicit-it=always, it would be good to support the more canonical -Wa,-mimplicit-it=always form as well. This is used by the Linux kernel when building a 32b THUMB2 image.

Otherwise, clang will fail when using the integrated assembler for inline asm, example:

In file included from arch/arm/mach-imx/pm-imx6.c:19:
In file included from ./include/linux/suspend.h:5:
In file included from ./include/linux/swap.h:9:
In file included from ./include/linux/memcontrol.h:13:
In file included from ./include/linux/cgroup.h:28:
In file included from ./include/linux/cgroup-defs.h:22:
In file included from ./include/linux/bpf-cgroup.h:5:
In file included from ./include/linux/bpf.h:21:
In file included from ./include/linux/kallsyms.h:12:
In file included from ./include/linux/mm.h:33:
In file included from ./include/linux/pgtable.h:6:
In file included from ./arch/arm/include/asm/pgtable.h:25:
./arch/arm/include/asm/tlbflush.h:342:2: error: predicated instructions must be in IT block
tlb_op(TLB_V7_UIS_FULL, "c8, c7, 0", zero);
^
./arch/arm/include/asm/tlbflush.h:320:30: note: expanded from macro 'tlb_op'
#define tlb_op(f, regs, arg) __tlb_op(f, "p15, 0, %0, " regs, arg)
^
./arch/arm/include/asm/tlbflush.h:314:21: note: expanded from macro '__tlb_op'
asm("tst %1, %2\n\t"
^
:2:2: note: instantiated into assembly here
mcrne p15, 0, r2, c8, c7, 0
^

[nickdesaulniers]

assigned to @nickdesaulniers

[DavidSpickett]

Reproducer:
$ cat /tmp/test.s
nopeq
$ ./bin/clang -target thumbv7a-none-linux-gnueabi -c -Wa,-mthumb /tmp/test.s
/tmp/test.s:1:1: error: predicated instructions must be in IT block
nopeq
^
$ ./bin/clang -target thumbv7a-none-linux-gnueabi -c -Wa,-mthumb -mimplicit-it=always /tmp/test.s
(which works because we pass -mllvm" "-arm-implicit-it=always" to cc1as)
$ ./bin/clang -target thumbv7a-none-linux-gnueabi -c -Wa,-mthumb -Wa,-mimplicit-it=always /tmp/test.s
clang-12: error: unsupported argument '-mimplicit-it=always' to option 'Wa,

Unlike -Wa,-mthumb we don't need to change the triple. All I need to do is make CollectArgsForIntegratedAssembler check for -Wa as well as the compiler argument.

case llvm::Triple::thumbeb:
if (Arg *A = Args.getLastArg(options::OPT_mimplicit_it_EQ)) {
StringRef Value = A->getValue();
if (Value == "always" || Value == "never" || Value == "arm" ||
Value == "thumb")

[nickdesaulniers]

https://reviews.llvm.org/D96285

[nickdesaulniers]

https://reviews.llvm.org/rGa680bc3a31d36d321ccf3801bdcff74d58842bfa