[llvm] [AIX] Handle arbitrary sized integers when lowering formal arguments passed on the stack (PR #149351)

[Amy Kwan via llvm-commits]

https://github.com/amy-kwan created https://github.com/llvm/llvm-project/pull/149351

When arbitrary sized (non-simple type, or non-power of two types) integers are passed on the stack, these integers are not handled when lowering formal arguments on AIX as we always assume we will encounter simple type integers.

However, it is possible for frontends to generate arbitrary sized immediate values in IR. Specifically in rustc, it will generate an integer value in LLVM IR for small structures that are less than a pointer size, which is done for optimization purposes for the Rust ABI. For example, if a Rust structure of three characters is passed into function on the stack,
```
struct my_struct {
  field1: u8,
  field2: u8,
  field3: u8,
}
```
This will generate an `i24` type in LLVM IR.

Currently, it is not obvious for the backend to distinguish an integer versus something that wasn't an integer to begin with (such as a struct), and the latter case would not have an extend on the parameter. Thus, this PR allows us to perform a truncation and extend on integers, both non-simple and simple types.



>From b31b4f5ccfe86b2fe552f21d201e6de0ec7f8179 Mon Sep 17 00:00:00 2001
From: Amy Kwan <amy.kwan1 at ibm.com>
Date: Thu, 17 Jul 2025 16:11:55 +0000
Subject: [PATCH] [AIX] Handle arbitrary sized integers when lowering formal
 arguments passed on the stack

---
 llvm/lib/Target/PowerPC/PPCISelLowering.cpp   | 14 ++++++--
 .../PowerPC/aix-lower-arbitrary-sized-ints.ll | 33 +++++++++++++++++++
 2 files changed, 45 insertions(+), 2 deletions(-)
 create mode 100644 llvm/test/CodeGen/PowerPC/aix-lower-arbitrary-sized-ints.ll

diff --git a/llvm/lib/Target/PowerPC/PPCISelLowering.cpp b/llvm/lib/Target/PowerPC/PPCISelLowering.cpp
index 459525ed4ee9a..13a29b78f0612 100644
--- a/llvm/lib/Target/PowerPC/PPCISelLowering.cpp
+++ b/llvm/lib/Target/PowerPC/PPCISelLowering.cpp
@@ -7296,9 +7296,19 @@ SDValue PPCTargetLowering::LowerFormalArguments_AIX(
       if (!ArgVT.isVector() && !ValVT.isVector() && ArgVT.isInteger() &&
           ValVT.isInteger() &&
           ArgVT.getScalarSizeInBits() < ValVT.getScalarSizeInBits()) {
+        // It is possible to have either real integer values that aren't
+        // the power of two sizes, or integers that were not originally
+        // integers. In the latter case, these could have came from structs,
+        // and these integers would not have an extend on the parameter.
+        // Since these types of integers do not have an extend specified
+        // in the first place, the type of extend that we do should not matter.
+        EVT TruncatedArgVT;
+        if (ArgVT.isSimple())
+          TruncatedArgVT = ArgVT.getSimpleVT() == MVT::i1 ? MVT::i8 : ArgVT;
+        else
+          TruncatedArgVT = ArgVT;
         SDValue ArgValueTrunc = DAG.getNode(
-            ISD::TRUNCATE, dl, ArgVT.getSimpleVT() == MVT::i1 ? MVT::i8 : ArgVT,
-            ArgValue);
+            ISD::TRUNCATE, dl, TruncatedArgVT, ArgValue);
         SDValue ArgValueExt =
             ArgSignExt ? DAG.getSExtOrTrunc(ArgValueTrunc, dl, ValVT)
                        : DAG.getZExtOrTrunc(ArgValueTrunc, dl, ValVT);
diff --git a/llvm/test/CodeGen/PowerPC/aix-lower-arbitrary-sized-ints.ll b/llvm/test/CodeGen/PowerPC/aix-lower-arbitrary-sized-ints.ll
new file mode 100644
index 0000000000000..297d2a3e10f94
--- /dev/null
+++ b/llvm/test/CodeGen/PowerPC/aix-lower-arbitrary-sized-ints.ll
@@ -0,0 +1,33 @@
+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py UTC_ARGS: --version 5
+; RUN: llc --verify-machineinstrs -mtriple powerpc-ibm-aix-xcoff \
+; RUN:   -mcpu=pwr8 < %s | FileCheck %s --check-prefixes=CHECK,CHECK32
+; RUN: llc --verify-machineinstrs -mtriple powerpc64-ibm-aix-xcoff \
+; RUN:   -mcpu=pwr8 < %s | FileCheck %s --check-prefixes=CHECK,CHECK64
+
+define ptr @lower_args(ptr %_0, i32 %0, i32 %1, i32 %2, i32 %3, ptr %4, ptr %5, i64 %6, i24 %7) {
+; CHECK-LABEL: lower_args:
+; CHECK:       # %bb.0: # %entry
+; CHECK-NEXT:    blr
+entry:
+  ret ptr %_0
+}
+
+define i32 @lower_args2(i32 %a, i32 %b, i32 %c, i32 %d, i32 %e, i32 %f, i32 %g, i32 %h, i24 %i) {
+; CHECK32-LABEL: lower_args2:
+; CHECK32:       # %bb.0: # %entry
+; CHECK32-NEXT:    lwz 3, 56(1)
+; CHECK32-NEXT:    addi 3, 3, 255
+; CHECK32-NEXT:    clrlwi 3, 3, 8
+; CHECK32-NEXT:    blr
+;
+; CHECK64-LABEL: lower_args2:
+; CHECK64:       # %bb.0: # %entry
+; CHECK64-NEXT:    lwz 3, 116(1)
+; CHECK64-NEXT:    addi 3, 3, 255
+; CHECK64-NEXT:    clrldi 3, 3, 40
+; CHECK64-NEXT:    blr
+entry:
+  %0 = add i24 %i, 255
+  %1 = zext i24 %0 to i32
+  ret i32 %1
+}