[ConstantFolding] Handle reading from type padding

[nikic]

ReadDataFromGlobal() did not handle reads from the padding of types (in the sense of type store size != type alloc size, rather than struct padding).

Return zero in that case.

Fixes #144279.

[llvmbot]

@llvm/pr-subscribers-llvm-analysis

@llvm/pr-subscribers-llvm-transforms

Author: Nikita Popov (nikic)

Changes

ReadDataFromGlobal() did not handle reads from the padding of types (in the sense of type store size != type alloc size, rather than struct padding).

Just bail out in that case. (Alternatively could make this return zero as well, but the value seems dubious.)

Fixes #144279.

---

Full diff: https://github.com/llvm/llvm-project/pull/144330.diff

2 Files Affected:

• (modified) llvm/lib/Analysis/ConstantFolding.cpp (+4)
• (modified) llvm/test/Transforms/InstSimplify/ConstProp/loads.ll (+38)

diff --git a/llvm/lib/Analysis/ConstantFolding.cpp b/llvm/lib/Analysis/ConstantFolding.cpp
index 2b7a438a9ef01..7bd67d2a95dbc 100644
--- a/llvm/lib/Analysis/ConstantFolding.cpp
+++ b/llvm/lib/Analysis/ConstantFolding.cpp
@@ -432,6 +432,10 @@ bool ReadDataFromGlobal(Constant *C, uint64_t ByteOffset, unsigned char *CurPtr,
   assert(ByteOffset <= DL.getTypeAllocSize(C->getType()) &&
          "Out of range access");
 
+  // Trying to read type padding.
+  if (ByteOffset >= DL.getTypeStoreSize(C->getType()))
+    return false;
+
   // If this element is zero or undefined, we can just return since *CurPtr is
   // zero initialized.
   if (isa<ConstantAggregateZero>(C) || isa<UndefValue>(C))
diff --git a/llvm/test/Transforms/InstSimplify/ConstProp/loads.ll b/llvm/test/Transforms/InstSimplify/ConstProp/loads.ll
index dd75560e25ced..134025514dcf6 100644
--- a/llvm/test/Transforms/InstSimplify/ConstProp/loads.ll
+++ b/llvm/test/Transforms/InstSimplify/ConstProp/loads.ll
@@ -441,3 +441,41 @@ define i128 @load-128bit(){
   %1 = load i128, ptr @global128, align 4
   ret i128 %1
 }
+
+
+@i40_struct = constant { i40, i8 } { i40 0, i8 1 }
+@i40_array = constant [2 x i40] [i40 0, i40 1]
+
+define i8 @load_i40_struct_padding() {
+; CHECK-LABEL: @load_i40_struct_padding(
+; CHECK-NEXT:    [[V:%.*]] = load i8, ptr getelementptr (i8, ptr @i40_struct, i64 6), align 1
+; CHECK-NEXT:    ret i8 [[V]]
+;
+  %v = load i8, ptr getelementptr (i8, ptr @i40_struct, i64 6)
+  ret i8 %v
+}
+
+define i16 @load_i40_struct_partial_padding() {
+; CHECK-LABEL: @load_i40_struct_partial_padding(
+; CHECK-NEXT:    ret i16 0
+;
+  %v = load i16, ptr getelementptr (i8, ptr @i40_struct, i64 4)
+  ret i16 %v
+}
+
+define i8 @load_i40_array_padding() {
+; CHECK-LABEL: @load_i40_array_padding(
+; CHECK-NEXT:    [[V:%.*]] = load i8, ptr getelementptr (i8, ptr @i40_array, i64 6), align 1
+; CHECK-NEXT:    ret i8 [[V]]
+;
+  %v = load i8, ptr getelementptr (i8, ptr @i40_array, i64 6)
+  ret i8 %v
+}
+
+define i16 @load_i40_array_partial_padding() {
+; CHECK-LABEL: @load_i40_array_partial_padding(
+; CHECK-NEXT:    ret i16 0
+;
+  %v = load i16, ptr getelementptr (i8, ptr @i40_array, i64 4)
+  ret i16 %v
+}

[dtcxzyw]

It is weird to me that the following load (across the padding) gets folded into 0.

@i40_struct = constant { i40, i8 } { i40 0, i8 1 }
define i16 @load_i40_struct_padding() {
  %v = load i16, ptr getelementptr (i8, ptr @i40_struct, i64 4) ; [4, 6)
  ret i16 %v
}

I think it is ok to always return zero, as the padding is initialized with undef.

[nikic]

Done.

[dtcxzyw]

LG. The PR title should be updated.