ARM: Align functions containing Thumb-2 jump tables to 4 bytes.

[pcc]

Summary

Functions with jump tables need an alignment of 4 because they use the ADR
instruction, which aligns the PC to 4 bytes before adding an offset.

[rengolin]

With the comment, LGTM. Thanks!

test/CodeGen/ARM/thumb-alignment.ll

25

Maybe add a check for ADR, to make sure you're not aligning for other reason (and to keep it clear why you're aligning to 4-bytes)?

[pcc]

test/CodeGen/ARM/thumb-alignment.ll

25

Done

lib/Target/ARM/ARMConstantIslandPass.cpp

bool ARMConstantIslands::runOnMachineFunction(MachineFunction &mf) {
 
   // Do the initial scan of the function, building up information about the
   // sizes of each block, the location of all the water, and finding all of the
   // constant pool users.
   initializeFunctionInfo(CPEMIs);
   CPEMIs.clear();
   DEBUG(dumpBBs());
 
+  // Functions with jump tables need an alignment of 4 because they use the ADR
+  // instruction, which aligns the PC to 4 bytes before adding an offset.
+  if (!T2JumpTables.empty())
+    MF->ensureAlignment(2);
 
   /// Remove dead constant pool entries.
   MadeChange |= removeUnusedCPEntries();
 
   // Iteratively place constant pool entries and fix up branches until there
   // is no change.
   unsigned NoCPIters = 0, NoBRIters = 0;
   while (true) {

test/CodeGen/ARM/thumb-alignment.ll

 ; RUN: llc -relocation-model=pic -mtriple=thumbv7-unknown-linux -o - %s | FileCheck %s
 
 @x = external global i32
 
 ; CHECK: .globl foo
 ; CHECK-NEXT: .align  2
 define i32* @foo() {
   ret i32* @x
 }
 
 ; CHECK: .globl bar
 ; CHECK-NEXT: .align  1
 define i32* @bar() {
   ret i32* zeroinitializer
 }
+
+@a = external global i32
+@b = external global i32
+@c = external global i32
+@d = external global i32
+
+; Create a Thumb-2 jump table, which should force alignment to 4 bytes.
+
+; CHECK: .globl baz
+; CHECK-NEXT: .align  2
+define i32 @baz() {
+  %1 = load i32, i32* @c, align 4
+  switch i32 %1, label %7 [
+    i32 1, label %2
+    i32 4, label %5
+    i32 9, label %5
+    i32 3, label %8
+  ]
+
+; <label>:2
+  %3 = load i32, i32* @a, align 4
+  %4 = tail call i32 bitcast (i32 (...)* @fn2 to i32 (i32 (...)*, i32, i32)*)(i32 (...)* bitcast (i32 ()* @baz to i32 (...)*), i32 0, i32 %3) #2
+  br label %8
+
+; <label>:5
+  %6 = load i32, i32* @d, align 4
+  store i32 %6, i32* @b, align 4
+  br label %8
+
+; <label>:7
+  br label %8
+
+; <label>:8
+  %e.0 = phi i32 [ 1, %7 ], [ 1, %2 ], [ 0, %0 ], [ 0, %5 ]
+  ret i32 %e.0
+}
+
+declare i32 @fn2(...)