[RISCV] Enable global merging by default (llvm#115495)

From the discussion at the round-table at the RISC-V Summit it was clear
people see cases where global merging would help. So the direction of
enabling it by default and iteratively working to enable it in more
cases or to improve the heuristics seems sensible. This patch tries to
make a minimal step in that direction.

Author: asb

llvm/lib/Target/RISCV/RISCVTargetMachine.cpp

@@ -474,7 +474,13 @@ bool RISCVPassConfig::addPreISel() {
     addPass(createBarrierNoopPass());
   }
 
-  if (EnableGlobalMerge == cl::BOU_TRUE) {
+  if ((TM->getOptLevel() != CodeGenOptLevel::None &&
+       EnableGlobalMerge == cl::BOU_UNSET) ||
+      EnableGlobalMerge == cl::BOU_TRUE) {
+    // FIXME: Like AArch64, we disable extern global merging by default due to
+    // concerns it might regress some workloads. Unlike AArch64, we don't
+    // currently support enabling the pass in an "OnlyOptimizeForSize" mode.
+    // Investigating and addressing both items are TODO.
     addPass(createGlobalMergePass(TM, /* MaxOffset */ 2047,
                                   /* OnlyOptimizeForSize */ false,
                                   /* MergeExternalByDefault */

llvm/test/CodeGen/RISCV/O3-pipeline.ll

@@ -74,6 +74,7 @@
 ; CHECK-NEXT:       Exception handling preparation
 ; CHECK-NEXT:     A No-Op Barrier Pass
 ; CHECK-NEXT:     FunctionPass Manager
+; CHECK-NEXT:       Merge internal globals
 ; CHECK-NEXT:       Dominator Tree Construction
 ; CHECK-NEXT:       Basic Alias Analysis (stateless AA impl)
 ; CHECK-NEXT:       Function Alias Analysis Results

llvm/test/CodeGen/RISCV/global-merge-minsize-smalldata-nonzero.ll

@@ -1,7 +1,7 @@
 ; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py
-; RUN: llc -mtriple=riscv64 -riscv-enable-global-merge -riscv-force-enable-global-merge-external-globals \
+; RUN: llc -mtriple=riscv64 -riscv-force-enable-global-merge-external-globals \
 ; RUN:   -verify-machineinstrs < %s | FileCheck %s -check-prefix=SMALL-DATA
-; RUN: llc -mtriple=riscv64 -riscv-enable-global-merge -global-merge-min-data-size=0 \
+; RUN: llc -mtriple=riscv64 -global-merge-min-data-size=0 \
 ; RUN:   -riscv-force-enable-global-merge-external-globals -verify-machineinstrs < %s \
 ; RUN:   | FileCheck %s -check-prefix=MINSIZE
 

llvm/test/CodeGen/RISCV/global-merge-minsize-smalldata-zero.ll

@@ -1,7 +1,7 @@
 ; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py
-; RUN: llc -mtriple=riscv32 -riscv-enable-global-merge -riscv-force-enable-global-merge-external-globals \
+; RUN: llc -mtriple=riscv32 -riscv-force-enable-global-merge-external-globals \
 ; RUN:   -verify-machineinstrs < %s | FileCheck %s -check-prefix=SMALL-DATA
-; RUN: llc -mtriple=riscv64 -riscv-enable-global-merge -global-merge-min-data-size=5 \
+; RUN: llc -mtriple=riscv64 -global-merge-min-data-size=5 \
 ; RUN:   -riscv-force-enable-global-merge-external-globals  -verify-machineinstrs < %s \
 ; RUN:   | FileCheck %s -check-prefix=MINSIZE
 

llvm/test/CodeGen/RISCV/global-merge-minsize.ll

@@ -1,7 +1,7 @@
 ; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py
-; RUN: llc -mtriple=riscv32 -riscv-enable-global-merge -riscv-force-enable-global-merge-external-globals \
+; RUN: llc -mtriple=riscv32 -riscv-force-enable-global-merge-external-globals \
 ; RUN:   -verify-machineinstrs < %s | FileCheck %s -check-prefix=RV32
-; RUN: llc -mtriple=riscv32 -riscv-enable-global-merge -global-merge-min-data-size=5 \
+; RUN: llc -mtriple=riscv32 -global-merge-min-data-size=5 \
 ; RUN:   -riscv-force-enable-global-merge-external-globals -verify-machineinstrs < %s | FileCheck %s -check-prefix=RV32-MINSIZE
 
 @ig1 = internal global i32 0, align 4

llvm/test/CodeGen/RISCV/global-merge-offset.ll

@@ -1,11 +1,11 @@
 ; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py
-; RUN: sed 's/ArrSize/100/g' %s | llc -mtriple=riscv32 -riscv-enable-global-merge \
+; RUN: sed 's/ArrSize/100/g' %s | llc -mtriple=riscv32 \
 ; RUN:   -riscv-force-enable-global-merge-external-globals -verify-machineinstrs | FileCheck %s
-; RUN: sed 's/ArrSize/100/g' %s | llc -mtriple=riscv64 -riscv-enable-global-merge \
+; RUN: sed 's/ArrSize/100/g' %s | llc -mtriple=riscv64 \
 ; RUN:   -riscv-force-enable-global-merge-external-globals -verify-machineinstrs | FileCheck %s
-; RUN: sed 's/ArrSize/101/g' %s | llc -mtriple=riscv32 -riscv-enable-global-merge \
+; RUN: sed 's/ArrSize/101/g' %s | llc -mtriple=riscv32 \
 ; RUN:   -riscv-force-enable-global-merge-external-globals -verify-machineinstrs | FileCheck %s -check-prefix=CHECK-TOOBIG
-; RUN: sed 's/ArrSize/101/g' %s | llc -mtriple=riscv64 -riscv-enable-global-merge \
+; RUN: sed 's/ArrSize/101/g' %s | llc -mtriple=riscv64 \
 ; RUN:   -riscv-force-enable-global-merge-external-globals -verify-machineinstrs | FileCheck %s -check-prefix=CHECK-TOOBIG
 
 ; This test demonstrates that the MaxOffset is set correctly for RISC-V by

llvm/test/CodeGen/RISCV/global-merge.ll

@@ -1,12 +1,10 @@
 ; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py
-; RUN: llc -mtriple=riscv32 -riscv-enable-global-merge -verify-machineinstrs < %s \
-; RUN:   | FileCheck %s
-; RUN: llc -mtriple=riscv64 -riscv-enable-global-merge -verify-machineinstrs < %s \
-; RUN:   | FileCheck %s
-; RUN: llc -mtriple=riscv32 -riscv-enable-global-merge \
+; RUN: llc -mtriple=riscv32 -verify-machineinstrs < %s | FileCheck %s
+; RUN: llc -mtriple=riscv64 -verify-machineinstrs < %s | FileCheck %s
+; RUN: llc -mtriple=riscv32 \
 ; RUN:     -riscv-force-enable-global-merge-external-globals -verify-machineinstrs < %s \
 ; RUN:   | FileCheck -check-prefix=CHECK-WEXTERN %s
-; RUN: llc -mtriple=riscv64 -riscv-enable-global-merge \
+; RUN: llc -mtriple=riscv64 \
 ; RUN:     -riscv-force-enable-global-merge-external-globals -verify-machineinstrs < %s \
 ; RUN:   | FileCheck -check-prefix=CHECK-WEXTERN %s