[llvm] Do not generate the special .ref for zero-sized sections (PR #66805)

[Wael Yehia via llvm-commits]

https://github.com/w2yehia created https://github.com/llvm/llvm-project/pull/66805

None

>From c205629c44a9bb27fdcac54a9f22bfd42b9353d1 Mon Sep 17 00:00:00 2001
From: Wael Yehia <wyehia at ca.ibm.com>
Date: Tue, 19 Sep 2023 13:49:39 -0400
Subject: [PATCH] Do not generate the special .ref for zero-sized sections

---
 llvm/lib/Target/PowerPC/PPCAsmPrinter.cpp | 26 ++++++++++++++++++-----
 1 file changed, 21 insertions(+), 5 deletions(-)

diff --git a/llvm/lib/Target/PowerPC/PPCAsmPrinter.cpp b/llvm/lib/Target/PowerPC/PPCAsmPrinter.cpp
index 4b97e3e1a09152f..826cfd4caa066ce 100644
--- a/llvm/lib/Target/PowerPC/PPCAsmPrinter.cpp
+++ b/llvm/lib/Target/PowerPC/PPCAsmPrinter.cpp
@@ -280,7 +280,7 @@ class PPCAIXAsmPrinter : public PPCAsmPrinter {
 
   void emitFunctionBodyEnd() override;
 
-  void emitPGORefs();
+  void emitPGORefs(Module &M);
 
   void emitEndOfAsmFile(Module &) override;
 
@@ -2652,10 +2652,26 @@ void PPCAIXAsmPrinter::emitFunctionEntryLabel() {
         getObjFileLowering().getFunctionEntryPointSymbol(Alias, TM));
 }
 
-void PPCAIXAsmPrinter::emitPGORefs() {
-  if (OutContext.hasXCOFFSection(
+void PPCAIXAsmPrinter::emitPGORefs(Module &M) {
+  if (!OutContext.hasXCOFFSection(
           "__llvm_prf_cnts",
-          XCOFF::CsectProperties(XCOFF::XMC_RW, XCOFF::XTY_SD))) {
+          XCOFF::CsectProperties(XCOFF::XMC_RW, XCOFF::XTY_SD)))
+    return;
+
+  // An XCOFF .ref directive translates into a relocation entry from the source
+  // (referring) csect to the destination (referred-to) csect. When the source
+  // csect has zero length, the relocation is ambiguous and should not be
+  // generated.
+  bool HasNonZeroLengthPrfCntsSection = false;
+  const DataLayout &DL = M.getDataLayout();
+  for (GlobalVariable &GV : M.globals())
+    if (GV.hasSection() && GV.getSection().equals("__llvm_prf_cnts") &&
+        DL.getTypeAllocSize(GV.getValueType()) > 0) {
+      HasNonZeroLengthPrfCntsSection = true;
+      break;
+    }
+
+  if (HasNonZeroLengthPrfCntsSection) {
     MCSection *CntsSection = OutContext.getXCOFFSection(
         "__llvm_prf_cnts", SectionKind::getData(),
         XCOFF::CsectProperties(XCOFF::XMC_RW, XCOFF::XTY_SD),
@@ -2689,7 +2705,7 @@ void PPCAIXAsmPrinter::emitEndOfAsmFile(Module &M) {
   if (M.empty() && TOCDataGlobalVars.empty())
     return;
 
-  emitPGORefs();
+  emitPGORefs(M);
 
   // Switch to section to emit TOC base.
   OutStreamer->switchSection(getObjFileLowering().getTOCBaseSection());