[GlobalMerge] Aggressively merge constants to reduce TOC entries (#111756)

Symbols that get mapped into the read-only section are loaded as part of
the text segment and will always need a TOC entry to be addressable. Add
an option to aggressively merge these read only globals to reduce TOC
usage.

Author: syzaara

llvm/lib/CodeGen/GlobalMerge.cpp

@@ -119,6 +119,11 @@ static cl::opt<bool> GlobalMergeGroupByUse(
     "global-merge-group-by-use", cl::Hidden,
     cl::desc("Improve global merge pass to look at uses"), cl::init(true));
 
+static cl::opt<bool> GlobalMergeAllConst(
+    "global-merge-all-const", cl::Hidden,
+    cl::desc("Merge all const globals without looking at uses"),
+    cl::init(false));
+
 static cl::opt<bool> GlobalMergeIgnoreSingleUse(
     "global-merge-ignore-single-use", cl::Hidden,
     cl::desc("Improve global merge pass to ignore globals only used alone"),
@@ -263,7 +268,7 @@ bool GlobalMergeImpl::doMerge(SmallVectorImpl<GlobalVariable *> &Globals,
       });
 
   // If we want to just blindly group all globals together, do so.
-  if (!GlobalMergeGroupByUse) {
+  if (!GlobalMergeGroupByUse || (GlobalMergeAllConst && isConst)) {
     BitVector AllGlobals(Globals.size());
     AllGlobals.set();
     return doMerge(Globals, AllGlobals, M, isConst, AddrSpace);

llvm/test/CodeGen/PowerPC/aix-aggressive-merge-const.ll

@@ -0,0 +1,93 @@
+; RUN: llc --verify-machineinstrs -mtriple powerpc-ibm-aix-xcoff \
+; RUN: -global-merge-all-const=true < %s | FileCheck %s
+
+; RUN: llc --verify-machineinstrs -mtriple powerpc-ibm-aix-xcoff \
+; RUN: -global-merge-all-const=false < %s | FileCheck --check-prefix=NOMERGE %s
+
+%struct.pc_t = type { i8 }
+%struct.S = type { i32, i32, i32, i32, [9 x i32] }
+
+@constinit = private unnamed_addr constant <{ i32, i32, i32, i32, [9 x i32] }> <{ i32 0, i32 0, i32 0, i32 2, [9 x i32] zeroinitializer }>, align 4
+@.str = private unnamed_addr constant [6 x i8] c"hello\00", align 1
+@.str.1 = private unnamed_addr constant [6 x i8] c"world\00", align 1
+@.str.2 = private unnamed_addr constant [6 x i8] c"abcde\00", align 1
+@.str.3 = private unnamed_addr constant [6 x i8] c"fghij\00", align 1
+@pc = internal constant %struct.pc_t zeroinitializer, align 1
+@s = internal constant %struct.S { i32 1, i32 2, i32 3, i32 4, [9 x i32] [i32 5, i32 6, i32 7, i32 8, i32 9, i32 10, i32 11, i32 12, i32 13] }, align 4
+
+; Function Attrs: mustprogress
+define noundef i32 @f5() {
+entry:
+  %call = tail call noundef i32 @f4(ptr noundef nonnull @pc)
+  ret i32 %call
+}
+
+declare noundef i32 @f4(ptr noundef)
+declare noundef i32 @printf(ptr nocapture noundef readonly, ...)
+
+define noundef i32 @f1() {
+entry:
+  %call = tail call i32 (ptr, ...) @printf(ptr noundef nonnull dereferenceable(1) @.str)
+  ret i32 %call
+}
+
+
+; Function Attrs: mustprogress nofree nounwind
+define noundef i32 @f2() {
+entry:
+  %call = tail call i32 (ptr, ...) @printf(ptr noundef nonnull dereferenceable(1) @.str.1)
+  ret i32 %call
+}
+
+define noundef i32 @f3() {
+entry:
+  %call = tail call i32 (ptr, ...) @printf(ptr noundef nonnull dereferenceable(1) @.str.2)
+  ret i32 %call
+}
+
+define noundef i32 @f7() {
+entry:
+  %call = tail call noundef i32 @f6(ptr noundef nonnull @s)
+  ret i32 %call
+}
+
+declare noundef i32 @f6(ptr noundef)
+
+; CHECK:             .csect L.._MergedGlobals[RO],2
+; CHECK-NEXT:        .lglobl pc                          # @_MergedGlobals
+; CHECK-NEXT:        .lglobl s
+; CHECK-NEXT:        .align  2
+; CHECK-NEXT:pc:
+; CHECK-NEXT:        .space  1
+; CHECK-NEXT:L...str:
+; CHECK-NEXT:        .string "hello"
+; CHECK-NEXT:L...str.1:
+; CHECK-NEXT:        .string "world"
+; CHECK-NEXT:L...str.2:
+; CHECK-NEXT:        .string "abcde"
+; CHECK-NEXT:L...str.3:
+; CHECK-NEXT:        .string "fghij"
+; CHECK-NEXT:        .space  3
+; CHECK-NEXT:L..constinit:
+; CHECK-NEXT:        .vbyte  4, 0                            # 0x0
+; CHECK-NEXT:        .vbyte  4, 0                            # 0x0
+; CHECK-NEXT:        .vbyte  4, 0                            # 0x0
+; CHECK-NEXT:        .vbyte  4, 2                            # 0x2
+; CHECK-NEXT:        .space  36
+; CHECK-NEXT:s:
+; CHECK-NEXT:        .vbyte  4, 1                            # 0x1
+; CHECK-NEXT:        .vbyte  4, 2                            # 0x2
+; CHECK-NEXT:        .vbyte  4, 3                            # 0x3
+; CHECK-NEXT:        .vbyte  4, 4                            # 0x4
+; CHECK-NEXT:        .vbyte  4, 5                            # 0x5
+; CHECK-NEXT:        .vbyte  4, 6                            # 0x6
+; CHECK-NEXT:        .vbyte  4, 7                            # 0x7
+; CHECK-NEXT:        .vbyte  4, 8                            # 0x8
+; CHECK-NEXT:        .vbyte  4, 9                            # 0x9
+; CHECK-NEXT:        .vbyte  4, 10                           # 0xa
+; CHECK-NEXT:        .vbyte  4, 11                           # 0xb
+; CHECK-NEXT:        .vbyte  4, 12                           # 0xc
+; CHECK-NEXT:        .vbyte  4, 13                           # 0xd
+
+
+; NOMERGE-NOT: L.._MergedGGlobals[RO]