Messages in this thread | | | Date | Mon, 24 Apr 2017 14:34:47 +0100 | From | Will Deacon <> | Subject | Re: [PATCH] arm64: prefetch: Change assembly to be compatible with gcc and clang |
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On Thu, Apr 20, 2017 at 09:42:07AM +0100, Mark Rutland wrote: > On Wed, Apr 19, 2017 at 02:22:11PM -0700, Matthias Kaehlcke wrote: > > clang fails to build with the current code: > > > > arch/arm64/include/asm/processor.h:172:15: error: invalid operand in > > inline asm: 'prfm pldl1keep, ${0:a}' > > > > Apparently clang does not support the 'a' modifier. Change the > > constraint from 'p' ('An operand that is a valid memory address is > > allowed') to 'Q' ('A memory address which uses a single base register > > with no offset'), which works for both gcc and clang. > > It looks like the current %a0 template and p constraint were inherited > from arch/arm, as they've been there from day one on arm64. > > Looking at the arch/arm history, the "a" operand modifier and "p" > constraint were introduced in commit: > > 16f719de62809e22 ("[ARM] 5196/1: fix inline asm constraints for preload") > > ... so as to avoid GCC assuming prefetch of a pointer implied it was not > NULL. Until that point, we'd used no operand modifier and "o" > constraint. > > It's not clear to me whether "o", "p", and "Q" constraints differ in > this regard on AArch64, or if the issue regarding NULL is still > relevant. The GCC docs say the "p" constraint is used for "a valid > memory address", which does sound like it shouldn't be NULL. > > Otherwise, this does look consistent with what we do elsewhere.
I really don't like using 'Q' here, for two reasons:
1. It means we likely allocate a register where we don't need to, because we're going to need to use [Xn] as the addressing mode, which means adding any immediate offsets.
2. As you mention, 16f719de62809e22 says that GCC will use this as an indication that the address is non-NULL.
We also can't just remove the 'a', because that will result in assembly failures. I haven't dug into exactly why, but I suspect it's because "p" can generate a label, which then won't assemble if surrounded by '[]'.
Will
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