[CHERIoT] Generate export table entries for weak function symbols.

These arise in newer libc++'s, where a library is used for explicit
instantiations of some std::string methods. These become weakly
exported libcall symbols from the library, and still need to be
present in the export table. Since they should only ever be materialized
in one translation unit, it's OK for the export table entry to be
strongly exported while the symbol itself is weakly exported.

Author: resistor

llvm/lib/Target/RISCV/RISCVAsmPrinter.cpp

@@ -742,10 +742,10 @@ void RISCVAsmPrinter::emitEndOfAsmFile(Module &M) {
           /*IsImport*/ false);
       auto Sym = C.getOrCreateSymbol(ExportName);
       OutStreamer->emitSymbolAttribute(Sym, MCSA_ELF_TypeObject);
-      // If the function isn't global, don't make its export table entry
-      // global either.  Two different compilation units in the same
-      // compartment may export different static things.
-      if (Entry.Fn.hasExternalLinkage() && !Entry.forceLocal)
+      // If the function isn't global, don't make its export table entry global
+      // either.  Two different compilation units in the same compartment may
+      // export different static things.
+      if (!Entry.Fn.isDiscardableIfUnused() && !Entry.forceLocal)
         OutStreamer->emitSymbolAttribute(Sym, MCSA_Global);
       OutStreamer->emitValueToAlignment(Align(4));
       OutStreamer->emitLabel(Sym);
@@ -771,7 +771,7 @@ void RISCVAsmPrinter::emitEndOfAsmFile(Module &M) {
         // Emit symbol alias in the export table for the alias using the same
         // attributes, linkage, and size as the primary entry.
         OutStreamer->emitSymbolAttribute(AliasExportSym, MCSA_ELF_TypeObject);
-        if (GA->hasExternalLinkage() && !Entry.forceLocal)
+        if (!GA->isDiscardableIfUnused() && !Entry.forceLocal)
           OutStreamer->emitSymbolAttribute(AliasExportSym, MCSA_Global);
         OutStreamer->emitAssignment(AliasExportSym,
                                     MCSymbolRefExpr::create(Sym, C));

llvm/test/CodeGen/RISCV/cheri/cheriot-global-weak.ll

@@ -0,0 +1,32 @@
+; RUN: llc --filetype=asm --mcpu=cheriot --mtriple=riscv32cheriot-unknown-cheriotrtos -target-abi cheriot  %s -mattr=+xcheri,+cap-mode,+xcheriot -o - | FileCheck %s
+target datalayout = "e-m:e-p:32:32-i64:64-n32-S128-pf200:64:64:64:32-A200-P200-G200"
+target triple = "riscv32cheriot-unknown-cheriotrtos"
+
+; Verify that we emit an export table entry for a weak function
+
+; CHECK: .section        .compartment_exports,"aR",@progbits
+; CHECK .type   __library_export_libcalls_foo,@object
+; CHECK: .p2align        2, 0x0
+; CHECK: __library_export_libcalls_foo:
+; CHECK: .half   foo-__compartment_pcc_start
+; CHECK: .byte   0
+; CHECK: .byte   0
+; CHECK: .size   __library_export_libcalls_foo, 4
+
+; Function Attrs: mustprogress nofree norecurse nosync nounwind willreturn memory(none)
+define weak dso_local cherilibcallcc noundef i32 @foo() local_unnamed_addr addrspace(200) #0 {
+entry:
+  ret i32 42
+}
+
+attributes #0 = { mustprogress nofree norecurse nosync nounwind willreturn memory(none) "no-trapping-math"="true" "stack-protector-buffer-size"="8" "target-cpu"="cheriot" "target-features"="+32bit,+c,+cap-mode,+e,+m,+relax,+xcheri,+zmmul,-a,-b,-d,-experimental-sdext,-experimental-sdtrig,-experimental-smctr,-experimental-ssctr,-experimental-svukte,-experimental-xqcia,-experimental-xqciac,-experimental-xqcicli,-experimental-xqcicm,-experimental-xqcics,-experimental-xqcicsr,-experimental-xqciint,-experimental-xqcilo,-experimental-xqcilsm,-experimental-xqcisls,-experimental-zalasr,-experimental-zicfilp,-experimental-zicfiss,-experimental-zvbc32e,-experimental-zvkgs,-f,-h,-i,-sha,-shcounterenw,-shgatpa,-shtvala,-shvsatpa,-shvstvala,-shvstvecd,-smaia,-smcdeleg,-smcsrind,-smdbltrp,-smepmp,-smmpm,-smnpm,-smrnmi,-smstateen,-ssaia,-ssccfg,-ssccptr,-sscofpmf,-sscounterenw,-sscsrind,-ssdbltrp,-ssnpm,-sspm,-ssqosid,-ssstateen,-ssstrict,-sstc,-sstvala,-sstvecd,-ssu64xl,-supm,-svade,-svadu,-svbare,-svinval,-svnapot,-svpbmt,-svvptc,-v,-xcheriot,-xcvalu,-xcvbi,-xcvbitmanip,-xcvelw,-xcvmac,-xcvmem,-xcvsimd,-xmipscmove,-xmipslsp,-xsfcease,-xsfvcp,-xsfvfnrclipxfqf,-xsfvfwmaccqqq,-xsfvqmaccdod,-xsfvqmaccqoq,-xsifivecdiscarddlone,-xsifivecflushdlone,-xtheadba,-xtheadbb,-xtheadbs,-xtheadcmo,-xtheadcondmov,-xtheadfmemidx,-xtheadmac,-xtheadmemidx,-xtheadmempair,-xtheadsync,-xtheadvdot,-xventanacondops,-xwchc,-za128rs,-za64rs,-zaamo,-zabha,-zacas,-zalrsc,-zama16b,-zawrs,-zba,-zbb,-zbc,-zbkb,-zbkc,-zbkx,-zbs,-zca,-zcb,-zcd,-zce,-zcf,-zcmop,-zcmp,-zcmt,-zdinx,-zfa,-zfbfmin,-zfh,-zfhmin,-zfinx,-zhinx,-zhinxmin,-zic64b,-zicbom,-zicbop,-zicboz,-ziccamoa,-ziccif,-zicclsm,-ziccrse,-zicntr,-zicond,-zicsr,-zifencei,-zihintntl,-zihintpause,-zihpm,-zimop,-zk,-zkn,-zknd,-zkne,-zknh,-zkr,-zks,-zksed,-zksh,-zkt,-ztso,-zvbb,-zvbc,-zve32f,-zve32x,-zve64d,-zve64f,-zve64x,-zvfbfmin,-zvfbfwma,-zvfh,-zvfhmin,-zvkb,-zvkg,-zvkn,-zvknc,-zvkned,-zvkng,-zvknha,-zvknhb,-zvks,-zvksc,-zvksed,-zvksg,-zvksh,-zvkt,-zvl1024b,-zvl128b,-zvl16384b,-zvl2048b,-zvl256b,-zvl32768b,-zvl32b,-zvl4096b,-zvl512b,-zvl64b,-zvl65536b,-zvl8192b" }
+
+!llvm.module.flags = !{!0, !1, !2, !4}
+!llvm.ident = !{!5}
+
+!0 = !{i32 1, !"wchar_size", i32 4}
+!1 = !{i32 1, !"target-abi", !"cheriot"}
+!2 = !{i32 6, !"riscv-isa", !3}
+!3 = !{!"rv32e2p0_m2p0_c2p0_zmmul1p0_xcheri0p0"}
+!4 = !{i32 8, !"SmallDataLimit", i32 0}
+!5 = !{!"clang version 20.1.3 ([email protected]:resistor/llvm-project-1.git b5150572190bf7926bc7c77858e6f6d49f49b0bb)"}