[IR] Do not assume that function pointers are aligned

[miyuki]

Summary

The value tracking analysis uses function alignment to infer that the
least significant bits of function pointers are known to be zero.
Unfortunately, this is not correct for ARM targets: the least
significant bit of a function pointer stores the ARM/Thumb state
information (i.e., the LSB is set for Thumb functions and cleared for
ARM functions).

The original approach (https://reviews.llvm.org/D44781) introduced a
new field for function pointer alignment in the DataLayout structure
to address this. But it seems unlikely that optimizations based on
function pointer alignment would bring much benefit in practice to
justify the additional maintenance burden, so this patch simply
assumes that function pointer alignment is always unknown.

[javed.absar]

lib/IR/Value.cpp

688 ↗

(On Diff #144085)

Maybe a good idea to comment this change a bit

[miyuki]

Added a comment

[efriedma]

LGTM

llvm/trunk/lib/IR/Value.cpp

uint64_t Value::getPointerDereferenceableBytes(const DataLayout &DL,
   return DerefBytes;
 }
 
 unsigned Value::getPointerAlignment(const DataLayout &DL) const {
   assert(getType()->isPointerTy() && "must be pointer");
 
   unsigned Align = 0;
   if (auto *GO = dyn_cast<GlobalObject>(this)) {
+    // Don't make any assumptions about function pointer alignment. Some
+    // targets use the LSBs to store additional information.
+    if (isa<Function>(GO))
+      return 0;
     Align = GO->getAlignment();
     if (Align == 0) {
       if (auto *GVar = dyn_cast<GlobalVariable>(GO)) {
         Type *ObjectType = GVar->getValueType();
         if (ObjectType->isSized()) {
           // If the object is defined in the current Module, we'll be giving
           // it the preferred alignment. Otherwise, we have to assume that it
           // may only have the minimum ABI alignment.

llvm/trunk/test/Analysis/ValueTracking/func-ptr-lsb.ll

+; RUN: opt -instcombine -S < %s | FileCheck %s
+
+target datalayout = "e-p:32:32-n32-S64"
+
+; CHECK-LABEL: @foo_ptr
+; CHECK: and
+define i32 @foo_ptr() {
+entry:
+  ; Even though the address of @foo is aligned, we cannot assume that the
+  ; pointer has the same alignment. This is not true for e.g. ARM targets
+  ; which store ARM/Thumb state in the LSB
+  ret i32 and (i32 ptrtoint (void ()* @foo to i32), i32 -4)
+}
+
+define internal void @foo() align 16 {
+entry:
+  ret void
+}