Look through address space casts in IsConstantOffsetFromGlobal()

[arichardson]

Summary

I originally needed this change for our fork of clang targeting CHERI which
uses address space 200 extensively. It turns out the only test that fails
after this change is Transforms/ConstProp/loads.ll which already contains
a FIXME for this case.

[arsenm]

I'm not sure this is correct because the addrspacecast could include an add of an additional constant offset or some other factor

[arichardson]

True, I didn't think of that. For us this will almost always work without adding an additional offset but I am not sure what the correct way to fix this is.

Would it make sense to add a function like canPerformConstantAddrSpaceCast(unsigned FromAS, unsigned ToAS) to DataLayout?

[arsenm]

In D41114#952977, @arichardson wrote:

True, I didn't think of that. For us this will almost always work without adding an additional offset but I am not sure what the correct way to fix this is.

Would it make sense to add a function like canPerformConstantAddrSpaceCast(unsigned FromAS, unsigned ToAS) to DataLayout?

I think there correct solution is to allow bitcasts between equivalently sized pointers for no-op casts

lib/Analysis/ConstantFolding.cpp

if ((GV = dyn_cast<GlobalValue>(C))) {
     Offset = APInt(BitWidth, 0);
     return true;
   }
 
   // Otherwise, if this isn't a constant expr, bail out.
   auto *CE = dyn_cast<ConstantExpr>(C);
   if (!CE) return false;
 
-  // Look through ptr->int and ptr->ptr casts.
+  // Look through ptr->int, ptr->ptr casts and address space casts.
   if (CE->getOpcode() == Instruction::PtrToInt ||
-      CE->getOpcode() == Instruction::BitCast)
+      CE->getOpcode() == Instruction::BitCast ||
+      CE->getOpcode() == Instruction::AddrSpaceCast)
     return IsConstantOffsetFromGlobal(CE->getOperand(0), GV, Offset, DL);
 
   // i32* getelementptr ([5 x i32]* @a, i32 0, i32 5)
   auto *GEP = dyn_cast<GEPOperator>(CE);
   if (!GEP)
     return false;
 
   unsigned BitWidth = DL.getPointerTypeSizeInBits(GEP->getType());

test/Transforms/ConstProp/loads.ll

 ; BE-LABEL: @test2(
 ; BE: ret i16 -8531
 }
 
 define i16 @test2_addrspacecast() {
   %r = load i16, i16 addrspace(1)* addrspacecast(i32* getelementptr ({{i32,i8},i32}, {{i32,i8},i32}* @g1, i32 0, i32 0, i32 0) to i16 addrspace(1)*)
   ret i16 %r
 
-; FIXME: Should be able to load through a constant addrspacecast.
 ; 0xBEEF
 ; LE-LABEL: @test2_addrspacecast(
-; XLE: ret i16 -16657
-; LE: load i16, i16 addrspace(1)* addrspacecast
+; LE: ret i16 -16657
 
 ; 0xDEAD
 ; BE-LABEL: @test2_addrspacecast(
-; XBE: ret i16 -8531
-; BE: load i16, i16 addrspace(1)* addrspacecast
+; BE: ret i16 -8531
 }
 
 ; Load of second 16 bits of 32-bit value.
 define i16 @test3() {
   %r = load i16, i16* getelementptr(i16, i16* bitcast(i32* getelementptr ({{i32,i8},i32}, {{i32,i8},i32}* @g1, i32 0, i32 0, i32 0) to i16*), i32 1)
   ret i16 %r
 
 ; 0xDEAD