Emit jump tables into .hot and .unlikely sections respectively

Author: mingmingl-llvm

llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp

@@ -2885,8 +2885,14 @@ void AsmPrinter::emitJumpTableInfo() {
     return;
   }
 
+  // When static data partitioning is enabled, collect jump table entries that
+  // go into the same section together to reduce the amount of section switch
+  // statements.
+  //
   // Iterate all jump tables, put hot jump table indices towards the beginning
-  // of the vector, and cold jump table indices towards the end.
+  // of the vector, and cold jump table indices towards the end. Meanwhile
+  // retain the relative orders of original jump tables within a hot or unlikely
+  // section by reversing the cold jump table indices.
   int NextHotJumpTableIndex = 0, NextColdJumpTableIndex = JT.size() - 1;
   JumpTableIndices.resize(JT.size());
   for (unsigned JTI = 0, JTSize = JT.size(); JTI < JTSize; ++JTI) {
@@ -2902,8 +2908,8 @@ void AsmPrinter::emitJumpTableInfo() {
         TLOF.getSectionForJumpTable(F, TM, &JT[0]), JTInDiffSection, *MJTI);
   }
 
-  if (NextHotJumpTableIndex != (int)JT.size()) {
-    // Retain the relative orders of original jump tables.
+  if (NextHotJumpTableIndex < (int)JT.size()) {
+    // Reverse the order of cold jump tables indices.
     for (int L = NextHotJumpTableIndex, R = JT.size() - 1; L < R; ++L, --R)
       std::swap(JumpTableIndices[L], JumpTableIndices[R]);
 

llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp

@@ -671,6 +671,8 @@ getELFSectionNameForGlobal(const GlobalObject *GO, SectionKind Kind,
     if (JTE) {
       if (JTE->Hotness == MachineFunctionDataHotness::Hot)
         raw_svector_ostream(Name) << ".hot";
+      else if (JTE->Hotness == MachineFunctionDataHotness::Cold)
+        raw_svector_ostream(Name) << ".unlikely";
     } else if (std::optional<StringRef> Prefix = F->getSectionPrefix()) {
       raw_svector_ostream(Name) << '.' << *Prefix;
       HasPrefix = true;

llvm/test/CodeGen/X86/jump-table-partition.ll

@@ -8,13 +8,12 @@
 
 ; When 'partition-static-data-sections' is enabled, static data splitter pass will
 ; categorize jump tables and assembly printer will place hot jump tables in the
-; `.hot`-suffixed read only section, and cold ones in the `.rodata` sections.
+; `.rodata.hot`-prefixed section, and cold ones in the `.rodata.unlikely`-prefixed section.
 ; Section names will optionally have `.<func>` if -function-sections is enabled.
-; RUN: llc -enable-split-machine-functions -partition-static-data-sections=true -function-sections=true -min-jump-table-entries=2  %s -o - 2>&1 | FileCheck %s --check-prefixes=FUNC,JT,HOT
+; RUN: llc -enable-split-machine-functions -partition-static-data-sections=true -function-sections=true -min-jump-table-entries=2  %s -o - 2>&1 | FileCheck %s --check-prefixes=FUNC,JT,DEFAULTHOT
 ; RUN: llc -enable-split-machine-functions -partition-static-data-sections=true -function-sections=false -min-jump-table-entries=2 %s -o - 2>&1 | FileCheck %s --check-prefixes=FUNCLESS,JT
 
-; Tests that jump tables with unknown hotness are categorized as cold if `-static-data-default-hotness` specifies so.
-; RUN: llc -enable-split-machine-functions -partition-static-data-sections=true -min-jump-table-entries=2 -static-data-default-hotness=cold -function-sections=true %s -o - 2>&1 | FileCheck %s --check-prefixes=FUNC,JT,DEFAULT
+; RUN: llc -enable-split-machine-functions -partition-static-data-sections=true -min-jump-table-entries=2 -static-data-default-hotness=cold -function-sections=true %s -o - 2>&1 | FileCheck %s --check-prefixes=FUNC,JT,DEFAULTCOLD
 
  ; Tests stat messages are expected.
 ; STAT-DAG: 2 static-data-splitter - Number of cold jump tables seen
@@ -27,15 +26,17 @@
 ; FUNCLESS: .section .rodata.hot,"a",@progbits
 ; JT: .LJTI0_0:
 ; JT: .LJTI0_2:
-; FUNC: .section .rodata.foo,"a",@progbits
-; FUNCLESS: .section .rodata,"a",@progbits
+; FUNC: .section .rodata.unlikely.foo,"a",@progbits
+; FUNCLESS: .section .rodata.unlikely,"a",@progbits
 ; JT: .LJTI0_1:
 ; JT: .LJTI0_3:
-; HOT: .section .rodata.hot.func_without_entry_count,"a",@progbits
-; HOT: .LJTI1_0:
+; DEFAULTHOT: .section .rodata.hot.func_without_entry_count,"a",@progbits
+; DEFAULTHOT: .LJTI1_0:
+; FUNCLESS: .section .rodata.hot,"a",@progbits
+; FUNCLESS: .LJTI1_0:
 
-; DEFAULT: .section .rodata.func_without_entry_count,"a",@progbits
-; DEFAULT: .LJTI1_0:
+; DEFAULTCOLD: .section .rodata.unlikely.func_without_entry_count,"a",@progbits
+; DEFAULTCOLD: .LJTI1_0:
 
 ; @foo has four jump tables, jt0, jt1, jt2 and jt3 in the input basic block
 ; order; jt0 and jt2 are hot, and jt1 and jt3 are cold.