[SimpliftyCFG] Put an alignment on generated switch tables

[dmgreen]

Summary

Previously the alignment on the newly created switch table data was not set,
meaning that DataLayout::getPreferredAlignment was free to overalign
it to 16 bytes. This causes unnecessary code bloat.

[efriedma]

LGTM

lib/Transforms/Utils/SimplifyCFG.cpp

SwitchLookupTable::SwitchLookupTable(
   // Store the table in an array.
   ArrayType *ArrayTy = ArrayType::get(ValueType, TableSize);
   Constant *Initializer = ConstantArray::get(ArrayTy, TableContents);
 
   Array = new GlobalVariable(M, ArrayTy, /*constant=*/true,
                              GlobalVariable::PrivateLinkage, Initializer,
                              "switch.table." + FuncName);
   Array->setUnnamedAddr(GlobalValue::UnnamedAddr::Global);
+  // Set the alignment to that of an array items. We will be only loading one
+  // value out of it.
+  Array->setAlignment(DL.getPrefTypeAlignment(ValueType));
   Kind = ArrayKind;
 }
 
 Value *SwitchLookupTable::BuildLookup(Value *Index, IRBuilder<> &Builder) {
   switch (Kind) {
   case SingleValueKind:
     return SingleValue;
   case LinearMapKind: {

test/Transforms/SimplifyCFG/X86/switch_to_lookup_table.ll

 ; RUN: opt < %s -simplifycfg -switch-to-lookup=true -keep-loops=false -S -mtriple=x86_64-unknown-linux-gnu | FileCheck %s
 
 target datalayout = "e-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v64:64:64-v128:128:128-a0:0:64-s0:64:64-f80:128:128-n8:16:32:64-S128"
 target triple = "x86_64-unknown-linux-gnu"
 
 ; The table for @f
-; CHECK: @switch.table.f = private unnamed_addr constant [7 x i32] [i32 55, i32 123, i32 0, i32 -1, i32 27, i32 62, i32 1]
+; CHECK: @switch.table.f = private unnamed_addr constant [7 x i32] [i32 55, i32 123, i32 0, i32 -1, i32 27, i32 62, i32 1], align 4
+
+; The char table for char
+; CHECK: @switch.table.char = private unnamed_addr constant [9 x i8] c"7{\00\FF\1B>\01!T", align 1
 
 ; The float table for @h
-; CHECK: @switch.table.h = private unnamed_addr constant [4 x float] [float 0x40091EB860000000, float 0x3FF3BE76C0000000, float 0x4012449BA0000000, float 0x4001AE1480000000]
+; CHECK: @switch.table.h = private unnamed_addr constant [4 x float] [float 0x40091EB860000000, float 0x3FF3BE76C0000000, float 0x4012449BA0000000, float 0x4001AE1480000000], align 4
 
 ; The table for @foostring
-; CHECK: @switch.table.foostring = private unnamed_addr constant [4 x i8*] [i8* getelementptr inbounds ([4 x i8], [4 x i8]* @.str, i64 0, i64 0), i8* getelementptr inbounds ([4 x i8], [4 x i8]* @.str1, i64 0, i64 0), i8* getelementptr inbounds ([4 x i8], [4 x i8]* @.str2, i64 0, i64 0), i8* getelementptr inbounds ([4 x i8], [4 x i8]* @.str3, i64 0, i64 0)]
+; CHECK: @switch.table.foostring = private unnamed_addr constant [4 x i8*] [i8* getelementptr inbounds ([4 x i8], [4 x i8]* @.str, i64 0, i64 0), i8* getelementptr inbounds ([4 x i8], [4 x i8]* @.str1, i64 0, i64 0), i8* getelementptr inbounds ([4 x i8], [4 x i8]* @.str2, i64 0, i64 0), i8* getelementptr inbounds ([4 x i8], [4 x i8]* @.str3, i64 0, i64 0)], align 8
 
 ; The table for @earlyreturncrash
-; CHECK: @switch.table.earlyreturncrash = private unnamed_addr constant [4 x i32] [i32 42, i32 9, i32 88, i32 5]
+; CHECK: @switch.table.earlyreturncrash = private unnamed_addr constant [4 x i32] [i32 42, i32 9, i32 88, i32 5], align 4
 
 ; The table for @large
 ; CHECK: @switch.table.large = private unnamed_addr constant [199 x i32] [i32 1, i32 4, i32 9,
 
 ; The table for @cprop
-; CHECK: @switch.table.cprop = private unnamed_addr constant [7 x i32] [i32 5, i32 42, i32 126, i32 -452, i32 128, i32 6, i32 7]
+; CHECK: @switch.table.cprop = private unnamed_addr constant [7 x i32] [i32 5, i32 42, i32 126, i32 -452, i32 128, i32 6, i32 7], align 4
 
 ; The table for @unreachable_case
-; CHECK: @switch.table.unreachable_case = private unnamed_addr constant [9 x i32] [i32 0, i32 0, i32 0, i32 2, i32 -1, i32 1, i32 1, i32 1, i32 1]
+; CHECK: @switch.table.unreachable_case = private unnamed_addr constant [9 x i32] [i32 0, i32 0, i32 0, i32 2, i32 -1, i32 1, i32 1, i32 1, i32 1], align 4
 
 ; A simple int-to-int selection switch.
 ; It is dense enough to be replaced by table lookup.
 ; The result is directly by a ret from an otherwise empty bb,
 ; so we return early, directly from the lookup bb.
 
 define i32 @f(i32 %c) {
 entry:

 ; CHECK: switch.lookup:
 ; CHECK-NEXT: %switch.gep = getelementptr inbounds [7 x i32], [7 x i32]* @switch.table.f, i32 0, i32 %switch.tableidx
 ; CHECK-NEXT: %switch.load = load i32, i32* %switch.gep
 ; CHECK-NEXT: ret i32 %switch.load
 ; CHECK: return:
 ; CHECK-NEXT: ret i32 15
 }
 
+; Same thing, but with i8's
+
+define i8 @char(i32 %c) {
+entry:
+  switch i32 %c, label %sw.default [
+    i32 42, label %return
+    i32 43, label %sw.bb1
+    i32 44, label %sw.bb2
+    i32 45, label %sw.bb3
+    i32 46, label %sw.bb4
+    i32 47, label %sw.bb5
+    i32 48, label %sw.bb6
+    i32 49, label %sw.bb7
+    i32 50, label %sw.bb8
+  ]
+
+sw.bb1: br label %return
+sw.bb2: br label %return
+sw.bb3: br label %return
+sw.bb4: br label %return
+sw.bb5: br label %return
+sw.bb6: br label %return
+sw.bb7: br label %return
+sw.bb8: br label %return
+sw.default: br label %return
+return:
+  %retval.0 = phi i8 [ 15, %sw.default ], [ 84, %sw.bb8 ], [ 33, %sw.bb7 ], [ 1, %sw.bb6 ], [ 62, %sw.bb5 ], [ 27, %sw.bb4 ], [ -1, %sw.bb3 ], [ 0, %sw.bb2 ], [ 123, %sw.bb1 ], [ 55, %entry ]
+  ret i8 %retval.0
+
+; CHECK-LABEL: @char(
+; CHECK: entry:
+; CHECK-NEXT: %switch.tableidx = sub i32 %c, 42
+; CHECK-NEXT: %0 = icmp ult i32 %switch.tableidx, 9
+; CHECK-NEXT: br i1 %0, label %switch.lookup, label %return
+; CHECK: switch.lookup:
+; CHECK-NEXT: %switch.gep = getelementptr inbounds [9 x i8], [9 x i8]* @switch.table.char, i32 0, i32 %switch.tableidx
+; CHECK-NEXT: %switch.load = load i8, i8* %switch.gep
+; CHECK-NEXT: ret i8 %switch.load
+; CHECK: return:
+; CHECK-NEXT: ret i8 15
+}
+
 ; A switch used to initialize two variables, an i8 and a float.
 
 declare void @dummy(i8 signext, float)
 define void @h(i32 %x) {
 entry:
   switch i32 %x, label %sw.default [
     i32 0, label %sw.epilog
     i32 1, label %sw.bb1