[llvm-dev] [cfe-dev] [RFC] ASM Goto With Output Constraints

Craig Topper via llvm-dev llvm-dev at lists.llvm.org
Thu Jun 27 15:32:18 PDT 2019


I believe at least some portion of the INLINEASM_BR decision is discussed
here  https://reviews.llvm.org/D53765?id=184024#inline-508610 Anything
that's on record anywhere should be in that review. I may have had some
conversations with Chandler on IRC, but I'm not sure.


~Craig


On Thu, Jun 27, 2019 at 2:53 PM Finkel, Hal J. <hfinkel at anl.gov> wrote:

>
> On 6/27/19 2:31 PM, Craig Topper via llvm-dev wrote:
>
> What about SelectionDAG representation? Currently we expand callbr to
> INLINEASM_BR and BR. Both of which are terminators. But in order to support
> outputs we would need to put CopyFromReg nodes between them.
>
>
> Or maybe we should support having terminators that define values? People
> ask about this from time to time, and that seems like the
> higher-overall-value extension to make to the MI representation.
>
>  -Hal
>
>
>
> ~Craig
>
>
> On Thu, Jun 27, 2019 at 12:18 PM Nick Desaulniers via cfe-dev <
> cfe-dev at lists.llvm.org> wrote:
>
>> + CBL mailing list
>>
>>
>> On Thu, Jun 27, 2019 at 11:08 AM Bill Wendling <isanbard at gmail.com>
>> wrote:
>>
>>> [Adding the correct cfe-dev mailing list address.]
>>>
>>> On Thu, Jun 27, 2019 at 11:06 AM Bill Wendling <isanbard at gmail.com>
>>> wrote:
>>>
>>>> Now that ASM goto support has landed, Nick Desaulniers and I wrote up a
>>>> document describing how to expand clang's implementation of ASM goto to
>>>> support output constraints. The work *should* be straight-forward, but
>>>> as always will need to be verified to work. Below is a copy of our
>>>> whitepaper. Please take a look and offer any comments you have.
>>>>
>>>> Share and enjoy!
>>>> -bw
>>>> Overview
>>>>
>>>> Support for asm goto
>>>> <https://gcc.gnu.org/onlinedocs/gcc/Extended-Asm.html> with output
>>>> constraints is a feature that the Linux community is interested in having. Adding
>>>> this new feature should give Clang a higher profile in the Linux community:
>>>>
>>>>
>>>>    -
>>>>
>>>>    It demonstrates the Clang community's commitment to supporting
>>>>    Linux.
>>>>    -
>>>>
>>>>    Developers are likely to adopt it on their own, which means they
>>>>    will need to use Clang in some fashion, either as a complete replacement
>>>>    for or in addition to GCC.
>>>>
>>>> Current state
>>>>
>>>> Clang's implementation of asm goto converts this code:
>>>>
>>>> int vogon(unsigned a, unsigned b) {   asm goto("poetry %0, %1" : : "r"(a),
>>>> "r"(b) : : error);   return a + b; error:   return -1; }
>>>>
>>>> into the following LLVM IR:
>>>>
>>>> define i32 @vogon(i32 %a, i32 %b) { entry:   callbr void asm sideeffect "poetry
>>>> $0, $1", "r,r,X"       (i32 %a, i32 %b, i8* blockaddress(@vogon,
>>>> %return))           to label %asm.fallthrough [label %return] asm
>>>> .fallthrough:   %add = add i32 %b, %a   br label %return return:
>>>> %retval.0 = phi i32 [ %add, %asm.fallthrough ], [ -1, %entry ]   ret
>>>> i32 %retval.0 }
>>>>
>>>> Our proposal won't change LLVM's current behavior–i.e. a callbr
>>>> without a return value will act in the same way as the current
>>>> implementation.
>>>> Proposal
>>>>
>>>> GCC restricts asm goto from having output constraints due to
>>>> limitations in its internal representation–i.e. GCC's control transfer
>>>> instructions cannot have outputs. For example:
>>>>
>>>> int vogon(int a, int b) {   asm goto("poetry %0, %1" : "=r"(a), "=r"(b)
>>>> : : : error);   return a + b; error:   return -1; }
>>>>
>>>> currently fails to compile in GCC with the following error:
>>>>
>>>> <source>: In function 'vogon': <source>:2:29: error: expected ':'
>>>> before string constant   2 |   asm goto("poetry %0, %1" : "=r"(a), "=r"(b)
>>>> : : : error);     |                         ^~~~~     |
>>>>           :
>>>>
>>>>
>>>>
>>>> ToT Clang matches GCC's behavior:
>>>>
>>>> <source>:2:30: error: 'asm goto' cannot have output constraints   asm
>>>> goto("poetry %0, %1" : "=r"(a), "=r"(b) : : : error);
>>>>
>>>> However, LLVM doesn't restrict control transfer instructions from
>>>> having outputs (e.g. the invoke instruction
>>>> <https://llvm.org/docs/LangRef.html#invoke-instruction>). We propose
>>>> changing LLVM's callbr instruction
>>>> <https://llvm.org/docs/LangRef.html#callbr-instruction> to allow
>>>> return values, similar to how LLVM's implementation of inline assembly (via
>>>> the call instruction
>>>> <https://llvm.org/docs/LangRef.html#call-instruction>) allows return
>>>> values. Since there can potentially be zero to many output constraints,
>>>> callbr would now return an aggregate which contains an element for
>>>> each output constraint.  These values would then be extracted via
>>>> extractvalue. With our proposal, the above C example will be converted
>>>> to LLVM IR like this:
>>>>
>>>> define i32 @vogon(i32 %a, i32 %b) { entry:   %0 = callbr { i32, i32 }
>>>> asm sideeffect "poetry $0, $1", "=r,=r,X"       (i8* blockaddress(
>>>> @vogon, %error))           to label %asm.fallthrough [label %error]
>>>>
>>>> asm.fallthrough:   %asmresult.a = extractvalue { i32, i32 } %0, 0
>>>> %asmresult.b = extractvalue { i32, i32 } %0, 1   %result = add i32
>>>> %asmresult.a, %asmresult.b   ret i32 %result error:   ret i32 -1 }
>>>>
>>>> Note that unlike the invoke instruction, callbr's return values are
>>>> assumed valid on all branches. The assumption is that the programmer
>>>> knows what their inline assembly is doing and where its output constraints
>>>> are valid. If the value isn't valid on a particular branch but is used
>>>> there anyway, then the result is a poison value. (Also, if a callbr's
>>>> return values affect a branch, it will be handled similarly to the
>>>> invoke instruction's implementation.) Here's an example of how this
>>>> would work:
>>>>
>>>> int vogon(int a, int b) {   asm goto("poetry %0, %1" : "=r"(a), "=r"(b)
>>>> : : : error);   if (a == 42)     return 42 * b;   return a + b; error:
>>>> return b - 42; }
>>>>
>>>> generates the following LLVM IR:
>>>>
>>>> define i32 @vogon(i32 %a, i32 %b) { entry:   %0 = callbr { i32, i32 }
>>>> asm sideeffect "poetry $0, $1", "=r,=r,X"       (i8* blockaddress(
>>>> @vogon, %error))           to label %asm.fallthrough [label %error] asm
>>>> .fallthrough:   %asmresult.a = extractvalue { i32, i32 } %0, 0
>>>> %tobool = icmp eq i32 %asmresult.a, 42   br i1 %tobool, label %if.true,
>>>> label %if.false  if.true:   %asmresult.b = extractvalue { i32, i32 } %0,
>>>> 1   %mul = mul i32 42, %asmresult.b   ret i32 %mul if.false:
>>>> %asmresult.a.1 = extractvalue { i32, i32 } %0, 0   %asmresult.b.1 =
>>>> extractvalue { i32, i32 } %0, 1   %result = add i32 %asmresult.a.1,
>>>> %asmresult.b.1   ret i32 %result error:   %asmresult.b.error =
>>>> extractvalue { i32, i32 } %0, 1   %error.result = sub i32
>>>> %asmresult.b.error, 42   ret i32 %error.result }
>>>> Implementation
>>>>
>>>> Because LLVM's invoke instruction is a terminating instruction that
>>>> may have return values, we can use it as a template for callbr's
>>>> changes. The new functionality lies mostly in modifying Clang's front-end.
>>>> In particular, we need to do the following:
>>>>
>>>>
>>>>    -
>>>>
>>>>    Remove all error checks restricting asm goto from returning values,
>>>>    and
>>>>    -
>>>>
>>>>    Generate the extractvalue instructions on callbr's branches.
>>>>
>>>>
>>>> LLVM's middle- and back-ends need to be audited to ensure there are no
>>>> restrictions on callbr returning a value. We expect all passes to Just
>>>> Work™ without modifications, but of course will be verified.
>>>>
>>>
>>
>> --
>> Thanks,
>> ~Nick Desaulniers
>> _______________________________________________
>> cfe-dev mailing list
>> cfe-dev at lists.llvm.org
>> https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
>>
>
> _______________________________________________
> LLVM Developers mailing listllvm-dev at lists.llvm.orghttps://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
> --
> Hal Finkel
> Lead, Compiler Technology and Programming Languages
> Leadership Computing Facility
> Argonne National Laboratory
>
>
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