Messages in this thread | | | From | Nick Desaulniers <> | Date | Thu, 25 Jul 2019 14:30:51 -0700 | Subject | Re: [PATCH v2] powerpc: slightly improve cache helpers |
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On Mon, Jul 22, 2019 at 10:58 AM Segher Boessenkool <segher@kernel.crashing.org> wrote: > > On Mon, Jul 22, 2019 at 10:21:07AM -0700, Nick Desaulniers wrote: > > On Sun, Jul 21, 2019 at 11:19 PM Segher Boessenkool > > <segher@kernel.crashing.org> wrote: > > > On Sun, Jul 21, 2019 at 07:41:40PM -0700, Nathan Chancellor wrote: > > > > On Sun, Jul 21, 2019 at 01:01:50PM -0500, Segher Boessenkool wrote: > > > > > On Sun, Jul 21, 2019 at 12:58:46AM -0700, Nathan Chancellor wrote: > > > > > > 0000017c clear_user_page: > > > > > > 17c: 94 21 ff f0 stwu 1, -16(1) > > > > > > 180: 38 80 00 80 li 4, 128 > > > > > > 184: 38 63 ff e0 addi 3, 3, -32 > > > > > > 188: 7c 89 03 a6 mtctr 4 > > > > > > 18c: 38 81 00 0f addi 4, 1, 15 > > > > > > 190: 8c c3 00 20 lbzu 6, 32(3) > > > > > > 194: 98 c1 00 0f stb 6, 15(1) > > > > > > 198: 7c 00 27 ec dcbz 0, 4 > > > > > > 19c: 42 00 ff f4 bdnz .+65524 > > > > > > > > > > Uh, yeah, well, I have no idea what clang tried here, but that won't > > > > > work. It's copying a byte from each target cache line to the stack, > > > > > and then does clears the cache line containing that byte on the stack. > > > > > > > > > > I *guess* this is about "Z" and not about "%y", but you'll have to ask > > > > > the clang people. > > > > > > > > > > Or it may be that they do not treat inline asm operands as lvalues > > > > > properly? That rings some bells. Yeah that looks like it. > > > > > > The code is > > > __asm__ __volatile__ ("dcbz %y0" : : "Z"(*(u8 *)addr) : "memory"); > > > > > > so yeah it looks like clang took that *(u8 *)addr as rvalue, and > > > stored that in stack, and then used *that* as memory. > > > > What's the %y modifier supposed to mean here? > > It prints a memory address for an indexed operand. > > If you write just "%0" it prints addresses that are a single register > as "0(r3)" instead of "0,r3". Some instructions do not allow offset > form. > > > addr is in the list of > > inputs, so what's wrong with using it as an rvalue? > > It seems to use *(u8 *)addr as rvalue. Asm operands are lvalues. It > matters a lot for memory operands.
Hmm...not sure that's specified behavior. Anyways, I've filed: https://bugs.llvm.org/show_bug.cgi?id=42762 to see if folks more familiar with LLVM's ppc backend have some more thoughts.
I recommend considering reverting commit 6c5875843b87 ("powerpc: slightly improve cache helpers") until the issue is resolved in clang, otherwise I'll probably just turn off our CI builds of PPC32 for the time being. -- Thanks, ~Nick Desaulniers
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