[RISCV] Use 'riscv-isa' module flag to set ELF flags and attributes. (#85155)

Walk all the ISA strings and set the subtarget bits for any extension we
find in any string.

This allows LTO output to have a ELF attributes from the union of all of
the files used to compile it.

Author: topperc

llvm/lib/Target/RISCV/RISCVAsmPrinter.cpp

@@ -100,7 +100,7 @@ class RISCVAsmPrinter : public AsmPrinter {
   bool emitDirectiveOptionArch();
 
 private:
-  void emitAttributes();
+  void emitAttributes(const MCSubtargetInfo &SubtargetInfo);
 
   void emitNTLHint(const MachineInstr *MI);
 
@@ -385,8 +385,32 @@ void RISCVAsmPrinter::emitStartOfAsmFile(Module &M) {
   if (const MDString *ModuleTargetABI =
           dyn_cast_or_null<MDString>(M.getModuleFlag("target-abi")))
     RTS.setTargetABI(RISCVABI::getTargetABI(ModuleTargetABI->getString()));
+
+  MCSubtargetInfo SubtargetInfo = *TM.getMCSubtargetInfo();
+
+  // Use module flag to update feature bits.
+  if (auto *MD = dyn_cast_or_null<MDNode>(M.getModuleFlag("riscv-isa"))) {
+    for (auto &ISA : MD->operands()) {
+      if (auto *ISAString = dyn_cast_or_null<MDString>(ISA)) {
+        auto ParseResult = llvm::RISCVISAInfo::parseArchString(
+            ISAString->getString(), /*EnableExperimentalExtension=*/true,
+            /*ExperimentalExtensionVersionCheck=*/true);
+        if (!errorToBool(ParseResult.takeError())) {
+          auto &ISAInfo = *ParseResult;
+          for (const auto &Feature : RISCVFeatureKV) {
+            if (ISAInfo->hasExtension(Feature.Key) &&
+                !SubtargetInfo.hasFeature(Feature.Value))
+              SubtargetInfo.ToggleFeature(Feature.Key);
+          }
+        }
+      }
+    }
+
+    RTS.setFlagsFromFeatures(SubtargetInfo);
+  }
+
   if (TM.getTargetTriple().isOSBinFormatELF())
-    emitAttributes();
+    emitAttributes(SubtargetInfo);
 }
 
 void RISCVAsmPrinter::emitEndOfAsmFile(Module &M) {
@@ -398,13 +422,13 @@ void RISCVAsmPrinter::emitEndOfAsmFile(Module &M) {
   EmitHwasanMemaccessSymbols(M);
 }
 
-void RISCVAsmPrinter::emitAttributes() {
+void RISCVAsmPrinter::emitAttributes(const MCSubtargetInfo &SubtargetInfo) {
   RISCVTargetStreamer &RTS =
       static_cast<RISCVTargetStreamer &>(*OutStreamer->getTargetStreamer());
   // Use MCSubtargetInfo from TargetMachine. Individual functions may have
   // attributes that differ from other functions in the module and we have no
   // way to know which function is correct.
-  RTS.emitTargetAttributes(*TM.getMCSubtargetInfo(), /*EmitStackAlign*/ true);
+  RTS.emitTargetAttributes(SubtargetInfo, /*EmitStackAlign*/ true);
 }
 
 void RISCVAsmPrinter::emitFunctionEntryLabel() {

llvm/test/CodeGen/RISCV/attributes-module-flag.ll

@@ -0,0 +1,17 @@
+; RUN: llc -mtriple=riscv32 %s -o - | FileCheck %s --check-prefix=RV32
+; RUN: llc -mtriple=riscv64 %s -o - | FileCheck %s --check-prefix=RV64
+
+; Test generation of ELF attribute from module metadata
+
+; RV32: .attribute 5, "rv32i2p1_m2p0_zba1p0"
+; RV64: .attribute 5, "rv64i2p1_m2p0_zba1p0"
+
+define i32 @addi(i32 %a) {
+  %1 = add i32 %a, 1
+  ret i32 %1
+}
+
+!llvm.module.flags = !{!0}
+
+!0 = !{i32 6, !"riscv-isa", !1}
+!1 = !{!"rv64i2p1_m2p0", !"rv64i2p1_zba1p0"}

llvm/test/CodeGen/RISCV/module-elf-flags.ll

@@ -0,0 +1,13 @@
+; RUN: llc -mtriple=riscv32 -filetype=obj < %s | llvm-readelf -h - | FileCheck -check-prefixes=FLAGS %s
+
+; FLAGS: Flags: 0x11, RVC, TSO
+
+define i32 @addi(i32 %a) {
+  %1 = add i32 %a, 1
+  ret i32 %1
+}
+
+!llvm.module.flags = !{!0}
+
+!0 = !{i32 6, !"riscv-isa", !1}
+!1 = !{!"rv64i2p1_c2p0_ztso0p1"}