[llvm] [PowerPC] Alignment of toc-data symbol should not be changed during optimization (PR #94593)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-ir

Author: Kai Luo (bzEq)

<details>
<summary>Changes</summary>

Currently, the alignment of toc-data symbol might be changed during instcombine
```
IC: Visiting:   %global = alloca %struct.widget, align 8                                                                                         
Found alloca equal to global:   %global = alloca %struct.widget, align 8                                                                         
  memcpy =   call void @<!-- -->llvm.memcpy.p0.p0.i64(ptr nonnull align 1 %global, ptr align 1 @<!-- -->global, i64 3, i1 false)
```
The `alloca` is created with `PrefAlign` which is 8 and after IC, the alignment of `@<!-- -->global` is enforced into `8`, same as the `alloca`. This is not expected, since toc-data symbol has the same alignment as toc entry and should not be increased during optimizations.

---
Full diff: https://github.com/llvm/llvm-project/pull/94593.diff


2 Files Affected:

- (modified) llvm/lib/IR/Globals.cpp (+8) 
- (modified) llvm/test/CodeGen/PowerPC/tocdata-firm-alignment.ll (+1-1) 


``````````diff
diff --git a/llvm/lib/IR/Globals.cpp b/llvm/lib/IR/Globals.cpp
index 6f071847bb58..2943a2aa0cf6 100644
--- a/llvm/lib/IR/Globals.cpp
+++ b/llvm/lib/IR/Globals.cpp
@@ -335,6 +335,14 @@ bool GlobalObject::canIncreaseAlignment() const {
   if (isELF && !isDSOLocal())
     return false;
 
+  bool isXCOFF =
+      (!Parent || Triple(Parent->getTargetTriple()).isOSBinFormatXCOFF());
+  if (isXCOFF)
+    if (const GlobalVariable *GV = dyn_cast<GlobalVariable>(this))
+      // GV with toc-data attribute is put in the region same as toc entry.
+      // Its alignment should be the same as toc entry.
+      return !GV->hasAttribute("toc-data");
+
   return true;
 }
 
diff --git a/llvm/test/CodeGen/PowerPC/tocdata-firm-alignment.ll b/llvm/test/CodeGen/PowerPC/tocdata-firm-alignment.ll
index c982713d4f8d..4ecec36bc977 100644
--- a/llvm/test/CodeGen/PowerPC/tocdata-firm-alignment.ll
+++ b/llvm/test/CodeGen/PowerPC/tocdata-firm-alignment.ll
@@ -5,7 +5,7 @@ target triple = "powerpc-ibm-aix7.2.0.0"
 
 %struct.widget = type { i8, i8, i8 }
 
-; CHECK: @global = {{.*}}constant %struct.widget { i8 4, i8 0, i8 0 }, align 8 #0
+; CHECK: @global = {{.*}}constant %struct.widget { i8 4, i8 0, i8 0 }, align 4 #0
 @global = constant %struct.widget { i8 4, i8 0, i8 0 }, align 4 #0
 
 define void @baz() #1 {

``````````

</details>


https://github.com/llvm/llvm-project/pull/94593