[inline asm] "=i" output constraint support - gcc compatiblity

[• zizhar]

Summary

https://bugs.llvm.org/show_bug.cgi?id=32027

In gcc, "i" is being ignored as an output constraint - when there's another char besides '+'\'=' and 'i', gcc ignores 'i' completely (just in output constraint).
In order to be gcc compatible, we need to ignore it as well.
The following patch ignores the 'i' and making sure things as "=i" are still not allowed.

('i' makes no sense as an output constraint but gcc allows it and ignores it.)

[ahatanak]

lib/Basic/TargetInfo.cpp

506

If we are going to ignore 'i', should we ignore other constraints for immediate operands too (for example, 'n', 'E", and 'F')?

[• zizhar]

lib/Basic/TargetInfo.cpp

506

I agree, we should.
I will add these cases aswell :)

[• zizhar]

Ping

[rnk]

lgtm

[blackliner]

I am getting errors with clang-tidy, is this the right place to ask for those additional constraints?

CUDA allows a few more output constraints --> https://docs.nvidia.com/cuda/inline-ptx-assembly/index.html#constraints

1.1.2. Constraints
There is a separate constraint letter for each PTX register type:

"h" = .u16 reg
"r" = .u32 reg
"l" = .u64 reg
"f" = .f32 reg
"d" = .f64 reg

lib/Basic/TargetInfo.cpp

case ',': // multiple alternative constraint.  Pass it.
         Name++;
       break;
     case '#': // Ignore as constraint.
       while (Name[1] && Name[1] != ',')
         Name++;
       break;
     case '?': // Disparage slightly code.
     case '!': // Disparage severely.
     case '*': // Ignore for choosing register preferences.
+    case 'i': // Ignore i as output constraint (match from the other chars)
       break;  // Pass them.
     }
 
     Name++;
   }
 
   // Early clobber with a read-write constraint which doesn't permit registers
   // is invalid.

test/Sema/asm.c

double test15() {
   __asm("0.0":"+*"(ret)); // expected-error {{invalid output constraint '+*' in asm}}
   __asm("0.0":"=%"(ret)); // expected-error {{invalid output constraint '=%' in asm}}
   __asm("0.0":"=,="(ret)); // expected-error {{invalid output constraint '=,=' in asm}}
   __asm("0.0":"=,g"(ret)); // no-error
   __asm("0.0":"=g"(ret)); // no-error
   return ret;
 }
 
+void iOutputConstraint(int x){
+  __asm ("nop" : "=ir" (x) : :); // no-error
+  __asm ("nop" : "=ri" (x) : :); // no-error
+  __asm ("nop" : "=ig" (x) : :); // no-error
+  __asm ("nop" : "=im" (x) : :); // no-error
+  __asm ("nop" : "=imr" (x) : :); // no-error
+  __asm ("nop" : "=i" (x) : :); // expected-error{{invalid output constraint '=i' in asm}}
+  __asm ("nop" : "+i" (x) : :); // expected-error{{invalid output constraint '+i' in asm}}
+  __asm ("nop" : "=ii" (x) : :); // expected-error{{invalid output constraint '=ii' in asm}}
+}
+
 // PR19837
 struct foo {
   int a;
 };
 register struct foo bar asm("esp"); // expected-error {{bad type for named register variable}}
 register float baz asm("esp"); // expected-error {{bad type for named register variable}}
 
 register int r0 asm ("edi"); // expected-error {{register 'edi' unsuitable for global register variables on this target}}