[OpenMP] [Flang] Resolved Issue #76121: Implemented Check for Unhandled Arguments in __kmpc_fork_call_if (#82221)

Root cause: Segmentation fault is caused by null pointer dereference
inside the __kmpc_fork_call_if function at
https://github.com/llvm/llvm-project/blob/main/openmp/runtime/src/z_Linux_asm.S#L1186
. __kmpc_fork_call_if is missing case to handle argc=0 .

Fix: Added a check inside the __kmp_invoke_microtask function to handle
the case when argc is 0.

---------

Co-authored-by: Singh <[email protected]>

Author: chandankds

openmp/runtime/src/z_Linux_asm.S

@@ -1150,6 +1150,9 @@ KMP_LABEL(kmp_invoke_pass_parms):	// put 1st - 6th parms to pkfn in registers.
 	movq	%rdi, %rbx	// pkfn -> %rbx
 	leaq	__gtid(%rbp), %rdi // &gtid -> %rdi (store 1st parm to pkfn)
 	leaq	__tid(%rbp), %rsi  // &tid -> %rsi (store 2nd parm to pkfn)
+	// Check if argc is 0
+	cmpq $0, %rax
+	je KMP_LABEL(kmp_no_args) // Jump ahead
 
 	movq	%r8, %r11	// p_argv -> %r11
 
@@ -1195,6 +1198,7 @@ KMP_LABEL(kmp_1_exit):
 	cmovnsq	(%r11), %rdx	// p_argv[0] -> %rdx (store 3rd parm to pkfn)
 #endif // KMP_MIC
 
+KMP_LABEL(kmp_no_args):
 	call	*%rbx		// call (*pkfn)();
 	movq	$1, %rax	// move 1 into return register;
 

openmp/runtime/test/misc_bugs/omp__kmpc_fork_call_if.c

@@ -0,0 +1,36 @@
+// RUN: %libomp-compile && %t | FileCheck %s
+
+#include <stdio.h>
+#include <omp.h>
+
+typedef int32_t kmp_int32;
+typedef void *ident_t;
+typedef void *kmpc_micro;
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+extern void __kmpc_fork_call_if(ident_t *loc, kmp_int32 argc,
+                                kmpc_micro microtask, kmp_int32 cond,
+                                void *args);
+#ifdef __cplusplus
+}
+#endif
+
+// Microtask function for parallel region
+void microtask(int *global_tid, int *bound_tid) {
+  // CHECK: PASS
+  if (omp_in_parallel()) {
+    printf("FAIL\n");
+  } else {
+    printf("PASS\n");
+  }
+}
+
+int main() {
+  // Condition for parallelization (false in this case)
+  int cond = 0;
+  // Call __kmpc_fork_call_if
+  __kmpc_fork_call_if(NULL, 0, microtask, cond, NULL);
+  return 0;
+}