[ConstantFolding] Fix handling of index width != pointer width

[nikic]

Per LangRef:

The offsets are then added to the low bits of the base address up to the index type width, with silently-wrapping two’s complement arithmetic. If the pointer size is larger than the index size, this means that the bits outside the index type width will not be affected.

The transform as implemented was doubly wrong, because it just truncated the original base pointer to the index width, losing the top bits entirely. Make sure we preserve the bits and use wrapping arithmetic within the low bits.

[llvmbot]

@llvm/pr-subscribers-llvm-analysis

@llvm/pr-subscribers-llvm-transforms

Author: Nikita Popov (nikic)

Changes

Per LangRef:

> The offsets are then added to the low bits of the base address up to the index type width, with silently-wrapping two’s complement arithmetic. If the pointer size is larger than the index size, this means that the bits outside the index type width will not be affected.

The transform as implemented was doubly wrong, because it just truncated the original base pointer to the index width, losing the top bits entirely. Make sure we preserve the bits and use wrapping arithmetic within the low bits.

---

Full diff: https://github.com/llvm/llvm-project/pull/130608.diff

2 Files Affected:

• (modified) llvm/lib/Analysis/ConstantFolding.cpp (+6-3)
• (added) llvm/test/Transforms/InstSimplify/ConstProp/inttoptr-gep-index-width.ll (+14)

diff --git a/llvm/lib/Analysis/ConstantFolding.cpp b/llvm/lib/Analysis/ConstantFolding.cpp
index d645bf8f7b621..f874d3d5e7318 100644
--- a/llvm/lib/Analysis/ConstantFolding.cpp
+++ b/llvm/lib/Analysis/ConstantFolding.cpp
@@ -943,18 +943,21 @@ Constant *SymbolicallyEvaluateGEP(const GEPOperator *GEP,
 
   // If the base value for this address is a literal integer value, fold the
   // getelementptr to the resulting integer value casted to the pointer type.
-  APInt BasePtr(BitWidth, 0);
+  APInt BasePtr(DL.getPointerTypeSizeInBits(Ptr->getType()), 0);
   if (auto *CE = dyn_cast<ConstantExpr>(Ptr)) {
     if (CE->getOpcode() == Instruction::IntToPtr) {
       if (auto *Base = dyn_cast<ConstantInt>(CE->getOperand(0)))
-        BasePtr = Base->getValue().zextOrTrunc(BitWidth);
+        BasePtr = Base->getValue().zextOrTrunc(BasePtr.getBitWidth());
     }
   }
 
   auto *PTy = cast<PointerType>(Ptr->getType());
   if ((Ptr->isNullValue() || BasePtr != 0) &&
       !DL.isNonIntegralPointerType(PTy)) {
-    Constant *C = ConstantInt::get(Ptr->getContext(), Offset + BasePtr);
+    // If the index size is smaller than the pointer size, add the the low
+    // bits only.
+    BasePtr.insertBits(BasePtr.trunc(BitWidth) + Offset, 0);
+    Constant *C = ConstantInt::get(Ptr->getContext(), BasePtr);
     return ConstantExpr::getIntToPtr(C, ResTy);
   }
 
diff --git a/llvm/test/Transforms/InstSimplify/ConstProp/inttoptr-gep-index-width.ll b/llvm/test/Transforms/InstSimplify/ConstProp/inttoptr-gep-index-width.ll
new file mode 100644
index 0000000000000..03056e8361e21
--- /dev/null
+++ b/llvm/test/Transforms/InstSimplify/ConstProp/inttoptr-gep-index-width.ll
@@ -0,0 +1,14 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 5
+; RUN: opt -S -passes=instsimplify < %s | FileCheck %s
+
+target datalayout = "p:16:16:16:8"
+
+; The GEP should only modify the low 8 bits of the pointer.
+define ptr @test() {
+; CHECK-LABEL: define ptr @test() {
+; CHECK-NEXT:    ret ptr inttoptr (i16 -256 to ptr)
+;
+  %base = inttoptr i16 -1 to ptr
+  %gep = getelementptr i8, ptr %base, i8 1
+  ret ptr %gep
+}

[dtcxzyw]

LG

[fhahn]

LGTM, thanks.

It looks like Alive2 might also need updating: https://alive2.llvm.org/ce/z/tf7ovb