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

Finkel, Hal J. via cfe-dev cfe-dev at lists.llvm.org
Thu Jun 27 14:14:37 PDT 2019

On 6/27/19 1:10 PM, Bill Wendling via cfe-dev 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.

This all sounds fairly straightforward and removes an technically-unnecessary restriction to produce a more-general capability - LLVM terminators can have return values, and so we have no problem representing the underlying concept. There is no governing standard here, and we made a fairly invasive change to LLVM already to support this extension in the first place. We should leverage that work to make the extension as useful as possible.


Share and enjoy!


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.


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 }


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.

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Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory
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