Reuse LLVM global constants. (#5273)

Reuse LLVM global constants: do not generate a new llvm::Global constant
for a pointer to a global constant for the same Carbon const_inst_id,
reuse already generated one.

toolchain/lower/file_context.cpp

@@ -154,9 +154,9 @@ auto FileContext::GetGlobal(SemIR::InstId inst_id,
     return const_value;
   }
 
-  // If we want a pointer to the constant, materialize a global to hold it.
-  // TODO: We could reuse the same global if the constant is used more than
-  // once.
+  if (auto result = global_variables().Lookup(const_inst_id)) {
+    return result.value();
+  }
 
   // Include both the name of the constant, if any, and the point of use in
   // the name of the variable.
@@ -177,10 +177,13 @@ auto FileContext::GetGlobal(SemIR::InstId inst_id,
   }
   llvm::StringRef sep = (use_name[0] == '.') ? "" : ".";
 
-  return new llvm::GlobalVariable(
+  auto* global_variable = new llvm::GlobalVariable(
       llvm_module(), GetType(sem_ir().GetPointeeType(value_rep.type_id)),
       /*isConstant=*/true, llvm::GlobalVariable::InternalLinkage, const_value,
       const_name + sep + use_name);
+
+  global_variables_.Insert(const_inst_id, global_variable);
+  return global_variable;
 }
 
 auto FileContext::GetOrCreateFunction(SemIR::FunctionId function_id,

toolchain/lower/testdata/class/virtual.carbon

@@ -110,8 +110,6 @@ fn Use() {
 // CHECK:STDOUT: @"_CIntermediate.Classes.$vtable" = external unnamed_addr constant ptr
 // CHECK:STDOUT: @"_CDerived.Classes.$vtable" = external unnamed_addr constant ptr
 // CHECK:STDOUT: @Base.val.loc7_3.2 = internal constant {} zeroinitializer
-// CHECK:STDOUT: @Base.val.loc8_44.6 = internal constant {} zeroinitializer
-// CHECK:STDOUT: @Base.val.loc9_48.6 = internal constant {} zeroinitializer
 // CHECK:STDOUT:
 // CHECK:STDOUT: define void @_CCreate.Create() !dbg !4 {
 // CHECK:STDOUT: entry:
@@ -125,13 +123,13 @@ fn Use() {
 // CHECK:STDOUT:   %.loc8_44.2.vptr = getelementptr inbounds nuw { ptr, {} }, ptr %i.var, i32 0, i32 0, !dbg !8
 // CHECK:STDOUT:   store ptr @"_CIntermediate.Classes.$vtable", ptr %.loc8_44.2.vptr, align 8, !dbg !8
 // CHECK:STDOUT:   %.loc8_44.5.base = getelementptr inbounds nuw { ptr, {} }, ptr %i.var, i32 0, i32 1, !dbg !8
-// CHECK:STDOUT:   call void @llvm.memcpy.p0.p0.i64(ptr align 1 %.loc8_44.5.base, ptr align 1 @Base.val.loc8_44.6, i64 0, i1 false), !dbg !8
+// CHECK:STDOUT:   call void @llvm.memcpy.p0.p0.i64(ptr align 1 %.loc8_44.5.base, ptr align 1 @Base.val.loc7_3.2, i64 0, i1 false), !dbg !8
 // CHECK:STDOUT:   call void @llvm.lifetime.start.p0(i64 8, ptr %d.var), !dbg !7
 // CHECK:STDOUT:   %.loc9_49.2.base = getelementptr inbounds nuw { { ptr, {} } }, ptr %d.var, i32 0, i32 0, !dbg !9
 // CHECK:STDOUT:   %.loc9_48.2.vptr = getelementptr inbounds nuw { ptr, {} }, ptr %.loc9_49.2.base, i32 0, i32 0, !dbg !10
 // CHECK:STDOUT:   store ptr @"_CDerived.Classes.$vtable", ptr %.loc9_48.2.vptr, align 8, !dbg !10
 // CHECK:STDOUT:   %.loc9_48.5.base = getelementptr inbounds nuw { ptr, {} }, ptr %.loc9_49.2.base, i32 0, i32 1, !dbg !10
-// CHECK:STDOUT:   call void @llvm.memcpy.p0.p0.i64(ptr align 1 %.loc9_48.5.base, ptr align 1 @Base.val.loc9_48.6, i64 0, i1 false), !dbg !10
+// CHECK:STDOUT:   call void @llvm.memcpy.p0.p0.i64(ptr align 1 %.loc9_48.5.base, ptr align 1 @Base.val.loc7_3.2, i64 0, i1 false), !dbg !10
 // CHECK:STDOUT:   call void @llvm.lifetime.start.p0(i64 8, ptr %d2.var), !dbg !7
 // CHECK:STDOUT:   ret void, !dbg !11
 // CHECK:STDOUT: }

toolchain/lower/testdata/global/class_with_fun.carbon

@@ -20,7 +20,6 @@ var a: A = {};
 // CHECK:STDOUT:
 // CHECK:STDOUT: @a = internal global {} zeroinitializer
 // CHECK:STDOUT: @A.val.loc13_12 = internal constant {} zeroinitializer
-// CHECK:STDOUT: @A.val.loc16_1 = internal constant {} zeroinitializer
 // CHECK:STDOUT: @llvm.global_ctors = appending global [1 x { i32, ptr, ptr }] [{ i32, ptr, ptr } { i32 0, ptr @_C__global_init.Main, ptr null }]
 // CHECK:STDOUT:
 // CHECK:STDOUT: define void @_Cret_a.Main(ptr sret({}) %return) !dbg !4 {
@@ -31,7 +30,7 @@ var a: A = {};
 // CHECK:STDOUT:
 // CHECK:STDOUT: define void @_C__global_init.Main() !dbg !8 {
 // CHECK:STDOUT: entry:
-// CHECK:STDOUT:   call void @llvm.memcpy.p0.p0.i64(ptr align 1 @a, ptr align 1 @A.val.loc16_1, i64 0, i1 false), !dbg !9
+// CHECK:STDOUT:   call void @llvm.memcpy.p0.p0.i64(ptr align 1 @a, ptr align 1 @A.val.loc13_12, i64 0, i1 false), !dbg !9
 // CHECK:STDOUT:   ret void, !dbg !10
 // CHECK:STDOUT: }
 // CHECK:STDOUT: