[llvm] [llvm] Use ABI instead of preferred alignment for const prop checks (PR #142500)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-transforms

Author: None (PiJoules)

<details>
<summary>Changes</summary>

We'd hit an assertion checking proper alignment for an i8 when building chromium because we used the prefered alignment (which is 4 bytes) instead of the ABI alignment (which is 1 byte). The ABI alignment should be used because that's the actual alignment needed to load a constant from the vtable.

This also updates the two `virtual-const-prop-small-alignment-*` to explicitly give ABI alignments for i64s.

---
Full diff: https://github.com/llvm/llvm-project/pull/142500.diff


3 Files Affected:

- (modified) llvm/lib/Transforms/IPO/WholeProgramDevirt.cpp (+1-1) 
- (modified) llvm/test/Transforms/WholeProgramDevirt/virtual-const-prop-small-alignment-32.ll (+1-1) 
- (modified) llvm/test/Transforms/WholeProgramDevirt/virtual-const-prop-small-alignment-64.ll (+1-1) 


``````````diff
diff --git a/llvm/lib/Transforms/IPO/WholeProgramDevirt.cpp b/llvm/lib/Transforms/IPO/WholeProgramDevirt.cpp
index 3a25255d0a4c8..a7d9f3ba24b24 100644
--- a/llvm/lib/Transforms/IPO/WholeProgramDevirt.cpp
+++ b/llvm/lib/Transforms/IPO/WholeProgramDevirt.cpp
@@ -1845,7 +1845,7 @@ bool DevirtModule::tryVirtualConstProp(
   if (BitWidth > 64)
     return false;
 
-  Align TypeAlignment = M.getDataLayout().getPrefTypeAlign(RetType);
+  Align TypeAlignment = M.getDataLayout().getABIIntegerTypeAlignment(BitWidth);
 
   // Make sure that each function is defined, does not access memory, takes at
   // least one argument, does not use its first argument (which we assume is
diff --git a/llvm/test/Transforms/WholeProgramDevirt/virtual-const-prop-small-alignment-32.ll b/llvm/test/Transforms/WholeProgramDevirt/virtual-const-prop-small-alignment-32.ll
index ab76f2c22e343..d501694bc4737 100644
--- a/llvm/test/Transforms/WholeProgramDevirt/virtual-const-prop-small-alignment-32.ll
+++ b/llvm/test/Transforms/WholeProgramDevirt/virtual-const-prop-small-alignment-32.ll
@@ -1,7 +1,7 @@
 ; RUN: opt -S -passes=wholeprogramdevirt -whole-program-visibility %s | FileCheck %s
 
 ;; This target uses 32-bit sized and aligned pointers.
-target datalayout = "e-p:32:32"
+target datalayout = "e-p:32:32-i64:64"
 
 ;; Constant propagation should be agnostic towards sections.
 ;; Also the new global should be in the original vtable's section.
diff --git a/llvm/test/Transforms/WholeProgramDevirt/virtual-const-prop-small-alignment-64.ll b/llvm/test/Transforms/WholeProgramDevirt/virtual-const-prop-small-alignment-64.ll
index c83fbc6ed5a19..b8e4a25bdc88d 100644
--- a/llvm/test/Transforms/WholeProgramDevirt/virtual-const-prop-small-alignment-64.ll
+++ b/llvm/test/Transforms/WholeProgramDevirt/virtual-const-prop-small-alignment-64.ll
@@ -1,7 +1,7 @@
 ; RUN: opt -S -passes=wholeprogramdevirt -whole-program-visibility %s | FileCheck %s
 
 ;; This target uses 64-bit sized and aligned pointers.
-target datalayout = "e-p:64:64"
+target datalayout = "e-p:64:64-i64:64"
 
 ;; Constant propagation should be agnostic towards sections.
 ;; Also the new global should be in the original vtable's section.

``````````

</details>


https://github.com/llvm/llvm-project/pull/142500