[flang] Add comdats to functions with linkonce linkage (#66516)

This fixes a bug where functions generated by the MLIR Math dialect, for
example ipowi, would fail to link with link.exe on Windows due to having
linkonce linkage but no associated comdat. Adding the comdat on ELF also
allows linkers to perform better garbage collection in the binary.

Simply adding comdats to all functions with this linkage type should
also cover future cases where linkonce or linkonce_odr functions might
be necessary.

Author: DavidTruby

flang/lib/Optimizer/CodeGen/CodeGen.cpp

@@ -34,6 +34,7 @@
 #include "mlir/Conversion/ReconcileUnrealizedCasts/ReconcileUnrealizedCasts.h"
 #include "mlir/Conversion/VectorToLLVM/ConvertVectorToLLVM.h"
 #include "mlir/Dialect/LLVMIR/LLVMDialect.h"
+#include "mlir/Dialect/LLVMIR/Transforms/AddComdats.h"
 #include "mlir/Dialect/OpenACC/OpenACC.h"
 #include "mlir/Dialect/OpenMP/OpenMPDialect.h"
 #include "mlir/IR/BuiltinTypes.h"
@@ -3861,6 +3862,14 @@ class FIRToLLVMLowering
                                                std::move(pattern)))) {
       signalPassFailure();
     }
+
+    // Run pass to add comdats to functions that have weak linkage on relevant platforms
+    if (fir::getTargetTriple(mod).supportsCOMDAT()) {
+      mlir::OpPassManager comdatPM("builtin.module");
+      comdatPM.addPass(mlir::LLVM::createLLVMAddComdats());
+      if (mlir::failed(runPipeline(comdatPM, mod)))
+        return signalPassFailure();
+    }
   }
 
 private:

flang/test/Fir/comdat.fir

@@ -0,0 +1,41 @@
+
+// RUN: fir-opt %s --fir-to-llvm-ir="target=x86_64-unknown-linux-gnu" | FileCheck %s --check-prefixes="CHECK-COMDAT"
+// RUN: fir-opt %s --fir-to-llvm-ir="target=x86_64-pc-windows-msvc" | FileCheck %s --check-prefixes="CHECK-COMDAT"
+// RUN: fir-opt %s --fir-to-llvm-ir="target=aarch64-apple-darwin" | FileCheck %s --check-prefixes="CHECK-NOCOMDAT"
+// RUN: fir-opt %s --fir-to-llvm-ir="target=powerpc64-ibm-aix" | FileCheck %s --check-prefixes="CHECK-NOCOMDAT"
+
+// CHECK-COMDAT: llvm.func linkonce @fun_linkonce(%arg0: i32) -> i32 comdat(@__llvm_comdat::@fun_linkonce)
+// CHECK-NOCOMDAT: llvm.func linkonce @fun_linkonce(%arg0: i32) -> i32 {
+func.func @fun_linkonce(%arg0: i32) -> i32 attributes {llvm.linkage = #llvm.linkage<linkonce>} {
+  return %arg0 : i32
+}
+
+// CHECK-COMDAT: llvm.func linkonce_odr @fun_linkonce_odr(%arg0: i32) -> i32 comdat(@__llvm_comdat::@fun_linkonce_odr)
+// CHECK-NOCOMDAT: llvm.func linkonce_odr @fun_linkonce_odr(%arg0: i32) -> i32 {
+func.func @fun_linkonce_odr(%arg0: i32) -> i32 attributes {llvm.linkage = #llvm.linkage<linkonce_odr>} {
+  return %arg0 : i32
+}
+
+// CHECK-COMDAT:  llvm.mlir.global linkonce constant @global_linkonce() comdat(@__llvm_comdat::@global_linkonce) {addr_space = 0 : i32} : i32 
+// CHECK-NOCOMDAT:  llvm.mlir.global linkonce constant @global_linkonce() {addr_space = 0 : i32} : i32  {
+fir.global linkonce @global_linkonce constant : i32 {
+  %0 = arith.constant 0 : i32
+  fir.has_value %0 : i32
+}
+
+// CHECK-COMDAT: llvm.comdat @__llvm_comdat {
+// CHECK-COMDAT: llvm.comdat_selector @fun_linkonce_odr any
+// CHECK-COMDAT: llvm.comdat_selector @fun_linkonce any
+// CHECK-COMDAT: llvm.comdat_selector @global_linkonce any
+// CHECK-COMDAT: llvm.comdat_selector @global_linkonce_odr any
+
+// CHECK-NOCOMDAT-NOT: llvm.comdat
+// CHECK-NOCOMDAT-NOT: llvm.comdat_selector
+
+
+// CHECK-COMDAT:  llvm.mlir.global linkonce_odr constant @global_linkonce_odr() comdat(@__llvm_comdat::@global_linkonce_odr) {addr_space = 0 : i32} : i32 
+// CHECK-NOCOMDAT:  llvm.mlir.global linkonce_odr constant @global_linkonce_odr() {addr_space = 0 : i32} : i32 { 
+fir.global linkonce_odr @global_linkonce_odr constant : i32 {
+  %0 = arith.constant 0 : i32
+  fir.has_value %0 : i32
+}

flang/test/Intrinsics/math-codegen.fir

@@ -1467,9 +1467,15 @@ func.func private @llvm.powi.f64.i32(f64, i32) -> f64
 func.func private @pow(f64, f64) -> f64
 
 //--- exponentiation_integer.fir
-// RUN: fir-opt %t/exponentiation_integer.fir --fir-to-llvm-ir="target=x86_64-unknown-linux-gnu" | FileCheck %t/exponentiation_integer.fir
+// RUN: fir-opt %t/exponentiation_integer.fir --fir-to-llvm-ir="target=x86_64-unknown-linux-gnu" | FileCheck %t/exponentiation_integer.fir --check-prefixes="CHECK,CHECK-COMDAT"
+// RUN: fir-opt %t/exponentiation_integer.fir --fir-to-llvm-ir="target=x86_64-pc-windows-msvc" | FileCheck %t/exponentiation_integer.fir --check-prefixes="CHECK,CHECK-COMDAT"
+// RUN: fir-opt %t/exponentiation_integer.fir --fir-to-llvm-ir="target=aarch64-apple-darwin" | FileCheck %t/exponentiation_integer.fir --check-prefixes="CHECK,CHECK-NOCOMDAT"
+// CHECK-COMDAT: llvm.comdat_selector @__mlir_math_ipowi_i32 any
+// CHECK-NOCOMDAT-NOT: llvm.comdat_selector @__mlir_math_ipowi_i32 any
 // CHECK: @_QPtest_int4
 // CHECK: llvm.call @__mlir_math_ipowi_i32({{%[A-Za-z0-9._]+}}, {{%[A-Za-z0-9._]+}}) : (i32, i32) -> i32
+// CHECK-COMDAT: llvm.func linkonce_odr @__mlir_math_ipowi_i32(%arg0: i32, %arg1: i32) -> i32 comdat(@__llvm_comdat::@__mlir_math_ipowi_i32)
+// CHECK-NOCOMDAT: llvm.func linkonce_odr @__mlir_math_ipowi_i32(%arg0: i32, %arg1: i32) -> i32
 
 func.func @_QPtest_int4(%arg0: !fir.ref<i32> {fir.bindc_name = "x"}, %arg1: !fir.ref<i32> {fir.bindc_name = "y"}, %arg2: !fir.ref<i32> {fir.bindc_name = "z"}) {
   %0 = fir.load %arg0 : !fir.ref<i32>