[llvm] 47ecd18 - Global string alignment (#142346)

[via llvm-commits]

Author: Dominik Steenken
Date: 2025-06-30T11:40:43+02:00
New Revision: 47ecd18f24d338d83c3a06dd1c7906b35edb8d91

URL: https://github.com/llvm/llvm-project/commit/47ecd18f24d338d83c3a06dd1c7906b35edb8d91
DIFF: https://github.com/llvm/llvm-project/commit/47ecd18f24d338d83c3a06dd1c7906b35edb8d91.diff

LOG: Global string alignment (#142346)

When creating global strings, some targets have requirements that need
to be taken into account. Previously, the global strings created by
`IRBuilder::createGlobalString` had a hard-coded alignment of `1`.

This commit makes it so that the alignment is taken from the data layout
instead, giving targets the chance to align global strings according to
their preferences.

This PR is motivated by (and should fix) #141491, where the 1-byte
alignment in a global string created by a `printf` optimization led to
the resulting assembly in a `-fno-PIC` compile to unexpectedly reference
the GOT based on whether the code contained `printf` statements of the
form `printf("foo\n");`.

Added: 
    llvm/test/Transforms/InstCombine/SystemZ/printf-opt-alignment.ll

Modified: 
    llvm/lib/IR/IRBuilder.cpp

Removed: 
    


################################################################################
diff  --git a/llvm/lib/IR/IRBuilder.cpp b/llvm/lib/IR/IRBuilder.cpp
index beefd5e7d7eef..28037d7ec5616 100644
--- a/llvm/lib/IR/IRBuilder.cpp
+++ b/llvm/lib/IR/IRBuilder.cpp
@@ -51,7 +51,7 @@ GlobalVariable *IRBuilderBase::CreateGlobalString(StringRef Str,
       *M, StrConstant->getType(), true, GlobalValue::PrivateLinkage,
       StrConstant, Name, nullptr, GlobalVariable::NotThreadLocal, AddressSpace);
   GV->setUnnamedAddr(GlobalValue::UnnamedAddr::Global);
-  GV->setAlignment(Align(1));
+  GV->setAlignment(M->getDataLayout().getPrefTypeAlign(getInt8Ty()));
   return GV;
 }
 

diff  --git a/llvm/test/Transforms/InstCombine/SystemZ/printf-opt-alignment.ll b/llvm/test/Transforms/InstCombine/SystemZ/printf-opt-alignment.ll
new file mode 100644
index 0000000000000..d01e759dd86a2
--- /dev/null
+++ b/llvm/test/Transforms/InstCombine/SystemZ/printf-opt-alignment.ll
@@ -0,0 +1,15 @@
+; RUN: opt < %s --passes=instcombine -S -mtriple=systemz-unknown | FileCheck %s
+;
+; Check that string replacements inserted by the instcombiner are properly aligned.
+; The specific case checked replaces `printf("foo\n")` with `puts("foo")`
+
+ at msg1 = constant [17 x i8] c"Alignment Check\0A\00", align 2
+; CHECK: c"Alignment Check\00", align 2
+
+; Function Attrs: noinline nounwind
+define dso_local void @foo() #0 {
+  %call = call signext i32 (ptr, ...) @printf(ptr noundef @msg1)
+  ret void
+}
+
+declare signext i32 @printf(ptr noundef, ...) #1