[llvm] [RISCV] Let -data-sections also work on sbss/sdata sections (PR #87040)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-backend-risc-v

Author: KaiWeng (KaiYG)

<details>
<summary>Changes</summary>

Add an unique suffix to .sbss/.sdata if -fdata-sections.
Without assigning an unique .sbss/.sdata section to each symbols, a linker may not be able to remove unused part when gc-section since all used and unused symbols are all mixed in the same .sbss/.sdata section.
I believe this also matches the behavior of gcc.

---
Full diff: https://github.com/llvm/llvm-project/pull/87040.diff


2 Files Affected:

- (modified) llvm/lib/Target/RISCV/RISCVTargetObjectFile.cpp (+30-4) 
- (added) llvm/test/CodeGen/RISCV/sdata-sections.ll (+17) 


``````````diff
diff --git a/llvm/lib/Target/RISCV/RISCVTargetObjectFile.cpp b/llvm/lib/Target/RISCV/RISCVTargetObjectFile.cpp
index e7c1a7e5d8bca1..b13df905249ab5 100644
--- a/llvm/lib/Target/RISCV/RISCVTargetObjectFile.cpp
+++ b/llvm/lib/Target/RISCV/RISCVTargetObjectFile.cpp
@@ -104,10 +104,36 @@ bool RISCVELFTargetObjectFile::isGlobalInSmallSection(
 MCSection *RISCVELFTargetObjectFile::SelectSectionForGlobal(
     const GlobalObject *GO, SectionKind Kind, const TargetMachine &TM) const {
   // Handle Small Section classification here.
-  if (Kind.isBSS() && isGlobalInSmallSection(GO, TM))
-    return SmallBSSSection;
-  if (Kind.isData() && isGlobalInSmallSection(GO, TM))
-    return SmallDataSection;
+  if (isGlobalInSmallSection(GO, TM)) {
+    // Emit a unique section for sdata/sbss when -fdata-section.
+    bool EmitUniquedSection = TM.getDataSections();
+
+    if (Kind.isBSS()) {
+      if (EmitUniquedSection) {
+        StringRef Prefix(".sbss");
+        SmallString<128> Name(Prefix);
+        Name.append(".");
+        Name.append(GO->getName());
+        return getContext().getELFSection(Name.str(), ELF::SHT_NOBITS,
+                                          ELF::SHF_WRITE | ELF::SHF_ALLOC);
+      }
+
+      return SmallBSSSection;
+    }
+
+    if (Kind.isData()) {
+      if (EmitUniquedSection) {
+        StringRef Prefix(".sdata");
+        SmallString<128> Name(Prefix);
+        Name.append(".");
+        Name.append(GO->getName());
+        return getContext().getELFSection(Name.str(), ELF::SHT_PROGBITS,
+                                          ELF::SHF_WRITE | ELF::SHF_ALLOC);
+      }
+
+      return SmallDataSection;
+    }
+  }
 
   // Otherwise, we work the same as ELF.
   return TargetLoweringObjectFileELF::SelectSectionForGlobal(GO, Kind, TM);
diff --git a/llvm/test/CodeGen/RISCV/sdata-sections.ll b/llvm/test/CodeGen/RISCV/sdata-sections.ll
new file mode 100644
index 00000000000000..175e6d744843a8
--- /dev/null
+++ b/llvm/test/CodeGen/RISCV/sdata-sections.ll
@@ -0,0 +1,17 @@
+; RUN: llc -mtriple=riscv32 -data-sections < %s | FileCheck -check-prefix=RV32 %s
+; RUN: llc -mtriple=riscv64 -data-sections < %s | FileCheck -check-prefix=RV64 %s
+
+; Append an unique name to each sdata/sbss section when -data-section.
+
+ at v = dso_local global i32 0, align 4
+ at r = dso_local global i64 7, align 8
+
+; SmallDataLimit set to 8, so we expect @v will be put in sbss
+; and @r will be put in sdata.
+!llvm.module.flags = !{!0}
+!0 = !{i32 8, !"SmallDataLimit", i32 8}
+
+; RV32:    .section        .sbss.v,"aw"
+; RV32:    .section        .sdata.r,"aw"
+; RV64:    .section        .sbss.v,"aw"
+; RV64:    .section        .sdata.r,"aw"

``````````

</details>


https://github.com/llvm/llvm-project/pull/87040