[inline asm] repeating constraint matches the same output

[• zizhar]

Summary

The problem is that in a case like that where the same variable is used twice to match to the same output:

__asm ("mov %2, %%eax\n\t"
       : "=a" (r0)
       : "0" (t0), "0" (t0));

Even though the inputs are both t0, we get the error "more than one input constraint matches the same output '1'"
gcc can handle these cases.

[ahatanak]

I tried compiling test18 with gcc7.1 (https://godbolt.org) and it fails to compile test18 with "inconsistent operand constraints in an 'asm'". Am I using the same version of the compiler as yours?

Also, do you need this to fix a real world case? I'm not sure I understand why or when anyone would need to use the same input operand twice.

[• zizhar]

I agree,
My bad, apparently gcc doesn't support that, I see no reason to add that.
Closing the review.

lib/Sema/SemaStmtAsm.cpp

   }
   SmallVector<size_t, 4> InputMatchedToOutput(OutputConstraintInfos.size(),
                                               ~0U);
+  SmallVector<StringRef, 4> InputMatchedToOutputIdName(
+      OutputConstraintInfos.size());
   for (unsigned i = 0, e = InputConstraintInfos.size(); i != e; ++i) {
     TargetInfo::ConstraintInfo &Info = InputConstraintInfos[i];
     StringRef ConstraintStr = Info.getConstraintStr();

     unsigned InputOpNo = i+NumOutputs;
     Expr *OutputExpr = Exprs[TiedTo];
     Expr *InputExpr = Exprs[InputOpNo];
+  StringRef InputIdentifierName;
 
+    // Extract the Identifier name in order to check that repeating constraints
+    // won't emit an error when they are the same
+    if (ImplicitCastExpr *implicitInputExpr =
+        dyn_cast<ImplicitCastExpr>(InputExpr))
+      if (DeclRefExpr *DeclRefInputExpr =
+          dyn_cast<DeclRefExpr>(implicitInputExpr->getSubExpr()))
+        InputIdentifierName =
+            DeclRefInputExpr->getDecl()->getIdentifier()->getName();
+
     // Make sure no more than one input constraint matches each output.
     assert(TiedTo < InputMatchedToOutput.size() && "TiedTo value out of range");
-    if (InputMatchedToOutput[TiedTo] != ~0U) {
+    if (InputMatchedToOutput[TiedTo] != ~0U &&
+        !InputIdentifierName.equals(InputMatchedToOutputIdName[TiedTo])) {
       Diag(NS->getInputExpr(i)->getLocStart(),
            diag::err_asm_input_duplicate_match)
           << TiedTo;

           << TiedTo;
       return StmtError();
     }
+
     InputMatchedToOutput[TiedTo] = i;
+    InputMatchedToOutputIdName[TiedTo] = InputIdentifierName;
 
     if (OutputExpr->isTypeDependent() || InputExpr->isTypeDependent())
       continue;

test/Sema/asm.c

           : "m" (test16_baz)); // expected-error {{reference to a global register variable in asm output with a memory constraint 'm'}}
 }
 
-int test17(int t0)
+int test17(int t0, int t1)
 {
   int r0, r1;
   __asm ("addl %2, %2\n\t"

          : "=a" (r0),
            "=&r" (r1)
          : "1" (t0),   // expected-note {{constraint '1' is already present here}}
-           "1" (t0));  // expected-error {{more than one input constraint matches the same output '1'}}
+           "1" (t1));  // expected-error {{more than one input constraint matches the same output '1'}}
   return r0 + r1;
 }
 
+int test18(int t0)
+{
+  int r0, r1;
+  __asm ("addl %2, %2\n\t"
+         "movl $123, %0"
+         : "=a" (r0),
+           "=&r" (r1)
+         : "1" (t0),   // no-error
+           "1" (t0));  // no-error
+  return r0 + r1;
+}
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