[flang] [llvm] [flang/flang-rt] Implement PERROR intrinsic form GNU Extension (PR #132406)

[Tom Eccles via llvm-commits]

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@@ -0,0 +1,14 @@
+! RUN: bbc -emit-fir %s -o - | FileCheck --check-prefixes=CHECK %s
+! RUN: %flang_fc1 -emit-fir %s -o - | FileCheck --check-prefixes=CHECK %s
+
+! CHECK-LABEL: func @_QPtest_perror(
+subroutine test_perror()
+  character(len=10) :: string
+  call perror(string)
+  ! CHECK: %[[C10:.*]] = arith.constant 10 : index
+  ! CHECK: %[[VAL_0:.*]] = fir.alloca !fir.char<1,10> {bindc_name = "string", uniq_name = "_QFtest_perrorEstring"}
+  ! CHECK: %[[VAL_1:.*]] = fir.declare %[[VAL_0]] typeparams %[[C10]] {uniq_name = "_QFtest_perrorEstring"} : (!fir.ref<!fir.char<1,10>>, index) -> !fir.ref<!fir.char<1,10>>
+  ! CHECK: %[[VAL_2:.*]] = fir.emboxchar %[[VAL_1]], %[[C10]] : (!fir.ref<!fir.char<1,10>>, index) -> !fir.boxchar<1>
+  ! CHECK: fir.call @_QPperror(%[[VAL_2]]) fastmath<contract> : (!fir.boxchar<1>) -> () 
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tblah wrote:

A fir.boxchar is a structure containing a pointer to the buffer and an i64 size. At least on my machine, this will generate the following LLVM-IR:
```
define void @test_perror_() #0 {
  %1 = alloca [10 x i8], i64 1, align 4
  %2 = insertvalue { ptr, i64 } undef, ptr %1, 0
  %3 = insertvalue { ptr, i64 } %2, i64 10, 1
  %4 = extractvalue { ptr, i64 } %3, 0
  %5 = extractvalue { ptr, i64 } %3, 1
  call void @perror_(ptr %4, i64 %5)
  ret void
}

declare void @perror_(ptr, i64) #1
```

I'm not sure whether it is guaranteed that the structure is passed like this no matter the ABI. @jeanPerier  do you know?

If this guaranteed then you need to add an unused length argument to your definition of perror so that the signature matches. Otherwise, you will have to wrap the intrinsic in lowering and add a `fir.box_addr` to get the address out of the `fir.boxchar`. 

https://github.com/llvm/llvm-project/pull/132406