[RISCV] Place mergeable small read only data into srodata section (#82214)

Small mergeable read only data was place on the sdata before, but it
also means it lose the mergeable property, which means lose some code
size optimization opportunity during link time.

Author: kito-cheng

llvm/lib/Target/RISCV/RISCVTargetObjectFile.cpp

@@ -32,6 +32,16 @@ void RISCVELFTargetObjectFile::Initialize(MCContext &Ctx,
       ".sdata", ELF::SHT_PROGBITS, ELF::SHF_WRITE | ELF::SHF_ALLOC);
   SmallBSSSection = getContext().getELFSection(".sbss", ELF::SHT_NOBITS,
                                                ELF::SHF_WRITE | ELF::SHF_ALLOC);
+  SmallRODataSection =
+      getContext().getELFSection(".srodata", ELF::SHT_PROGBITS, ELF::SHF_ALLOC);
+  SmallROData4Section = getContext().getELFSection(
+      ".srodata.cst4", ELF::SHT_PROGBITS, ELF::SHF_ALLOC | ELF::SHF_MERGE, 4);
+  SmallROData8Section = getContext().getELFSection(
+      ".srodata.cst8", ELF::SHT_PROGBITS, ELF::SHF_ALLOC | ELF::SHF_MERGE, 8);
+  SmallROData16Section = getContext().getELFSection(
+      ".srodata.cst16", ELF::SHT_PROGBITS, ELF::SHF_ALLOC | ELF::SHF_MERGE, 16);
+  SmallROData32Section = getContext().getELFSection(
+      ".srodata.cst32", ELF::SHT_PROGBITS, ELF::SHF_ALLOC | ELF::SHF_MERGE, 32);
 }
 
 const MCExpr *RISCVELFTargetObjectFile::getIndirectSymViaGOTPCRel(
@@ -126,8 +136,19 @@ bool RISCVELFTargetObjectFile::isConstantInSmallSection(
 MCSection *RISCVELFTargetObjectFile::getSectionForConstant(
     const DataLayout &DL, SectionKind Kind, const Constant *C,
     Align &Alignment) const {
-  if (isConstantInSmallSection(DL, C))
-    return SmallDataSection;
+  if (isConstantInSmallSection(DL, C)) {
+    if (Kind.isMergeableConst4())
+      return SmallROData4Section;
+    if (Kind.isMergeableConst8())
+      return SmallROData8Section;
+    if (Kind.isMergeableConst16())
+      return SmallROData16Section;
+    if (Kind.isMergeableConst32())
+      return SmallROData32Section;
+    // LLVM only generate up to .rodata.cst32, and use .rodata section if more
+    // than 32 bytes, so just use .srodata here.
+    return SmallRODataSection;
+  }
 
   // Otherwise, we work the same as ELF.
   return TargetLoweringObjectFileELF::getSectionForConstant(DL, Kind, C,

llvm/lib/Target/RISCV/RISCVTargetObjectFile.h

@@ -16,6 +16,11 @@ namespace llvm {
 /// This implementation is used for RISC-V ELF targets.
 class RISCVELFTargetObjectFile : public TargetLoweringObjectFileELF {
   MCSection *SmallDataSection;
+  MCSection *SmallRODataSection;
+  MCSection *SmallROData4Section;
+  MCSection *SmallROData8Section;
+  MCSection *SmallROData16Section;
+  MCSection *SmallROData32Section;
   MCSection *SmallBSSSection;
   unsigned SSThreshold = 8;
 

llvm/test/CodeGen/RISCV/srodata.ll

@@ -0,0 +1,47 @@
+; RUN: sed 's/SMALL_DATA_LIMIT/0/g' %s | \
+; RUN:   llc -mtriple=riscv32 -mattr=+d | \
+; RUN:   FileCheck -check-prefix=CHECK-SDL-0 %s
+; RUN: sed 's/SMALL_DATA_LIMIT/0/g' %s | \
+; RUN:   llc -mtriple=riscv64 -mattr=+d | \
+; RUN:   FileCheck -check-prefix=CHECK-SDL-0 %s
+; RUN: sed 's/SMALL_DATA_LIMIT/4/g' %s | \
+; RUN:   llc -mtriple=riscv32 -mattr=+d | \
+; RUN:   FileCheck -check-prefix=CHECK-SDL-4 %s
+; RUN: sed 's/SMALL_DATA_LIMIT/4/g' %s | \
+; RUN:   llc -mtriple=riscv64 -mattr=+d | \
+; RUN:   FileCheck -check-prefix=CHECK-SDL-4 %s
+; RUN: sed 's/SMALL_DATA_LIMIT/8/g' %s | \
+; RUN:   llc -mtriple=riscv32 -mattr=+d | \
+; RUN:   FileCheck -check-prefix=CHECK-SDL-8 %s
+; RUN: sed 's/SMALL_DATA_LIMIT/8/g' %s | \
+; RUN:   llc -mtriple=riscv64 -mattr=+d | \
+; RUN:   FileCheck -check-prefix=CHECK-SDL-8 %s
+; RUN: sed 's/SMALL_DATA_LIMIT/16/g' %s | \
+; RUN:   llc -mtriple=riscv32 -mattr=+d | \
+; RUN:   FileCheck -check-prefix=CHECK-SDL-16 %s
+; RUN: sed 's/SMALL_DATA_LIMIT/16/g' %s | \
+; RUN:   llc -mtriple=riscv64 -mattr=+d | \
+; RUN:   FileCheck -check-prefix=CHECK-SDL-16 %s
+
+define dso_local float @foof() {
+entry:
+  ret float 0x400A08ACA0000000
+}
+
+define dso_local double @foo() {
+entry:
+  ret double 0x400A08AC91C3E242
+}
+
+!llvm.module.flags = !{!0}
+
+!0 = !{i32 8, !"SmallDataLimit", i32 SMALL_DATA_LIMIT}
+
+; CHECK-SDL-0-NOT:    .section        .srodata.cst4
+; CHECK-SDL-0-NOT:    .section        .srodata.cst8
+; CHECK-SDL-4:        .section        .srodata.cst4
+; CHECK-SDL-4-NOT:    .section        .srodata.cst8
+; CHECK-SDL-8:        .section        .srodata.cst4
+; CHECK-SDL-8:        .section        .srodata.cst8
+; CHECK-SDL-16:       .section        .srodata.cst4
+; CHECK-SDL-16:       .section        .srodata.cst8