[MLIR][mlir-link] Fix weak linkage for two weak definitions (#48)

Mimics the behaviour of llvm-link

Author: Jezurko

mlir/include/mlir/Linker/LLVMLinkerMixin.h

@@ -293,10 +293,9 @@ class LLVMLinkerMixin {
     if (isLinkOnceLinkage(srcLinkage))
       return ConflictResolution::LinkFromDst;
 
-    if (isLinkOnceLinkage(dstLinkage) || isWeakLinkage(dstLinkage))
-      return ConflictResolution::LinkFromSrc;
-
     if (isCommonLinkage(srcLinkage)) {
+      if (isLinkOnceLinkage(dstLinkage) || isWeakLinkage(dstLinkage))
+        return ConflictResolution::LinkFromSrc;
       if (!isCommonLinkage(dstLinkage))
         return ConflictResolution::LinkFromDst;
       if (derived.getBitWidth(pair.src) > derived.getBitWidth(pair.dst))
@@ -309,10 +308,9 @@ class LLVMLinkerMixin {
       assert(!isAvailableExternallyLinkage(dstLinkage));
       if (isLinkOnceLinkage(dstLinkage) && isWeakLinkage(srcLinkage)) {
         return ConflictResolution::LinkFromSrc;
-      } else {
-        // No need to link the `src`
-        return ConflictResolution::LinkFromDst;
       }
+      // No need to link the `src`
+      return ConflictResolution::LinkFromDst;
     }
 
     if (isWeakForLinker(dstLinkage)) {

mlir/test/mlir-link/weak-linkage-e.mlir

@@ -0,0 +1,19 @@
+// RUN: mlir-link -split-input-file %s | FileCheck %s
+
+// CHECK: llvm.mlir.global weak @v(0 : i8)
+
+llvm.mlir.global weak @v(0 : i8) {addr_space = 0 : i32} : i8
+
+llvm.func @use_v1() -> !llvm.ptr {
+  %0 = llvm.mlir.addressof @v : !llvm.ptr
+  llvm.return %0 : !llvm.ptr
+}
+
+// -----
+
+llvm.mlir.global weak @v(1 : i8) {addr_space = 0 : i32} : i8
+
+llvm.func @use_v2() -> !llvm.ptr {
+  %0 = llvm.mlir.addressof @v : !llvm.ptr
+  llvm.return %0 : !llvm.ptr
+}

mlir/test/mlir-link/weak-linkage-f.mlir

@@ -0,0 +1,19 @@
+// RUN: mlir-link -split-input-file %s | FileCheck %s
+
+// CHECK: llvm.mlir.global weak @v(1 : i8)
+
+llvm.mlir.global weak @v(1 : i8) {addr_space = 0 : i32} : i8
+
+llvm.func @use_v2() -> !llvm.ptr {
+  %0 = llvm.mlir.addressof @v : !llvm.ptr
+  llvm.return %0 : !llvm.ptr
+}
+
+// -----
+
+llvm.mlir.global weak @v(0 : i8) {addr_space = 0 : i32} : i8
+
+llvm.func @use_v1() -> !llvm.ptr {
+  %0 = llvm.mlir.addressof @v : !llvm.ptr
+  llvm.return %0 : !llvm.ptr
+}