[llvm] [EarlyCSE] Rematerialize alignment assumption. (PR #109131)

[Florian Hahn via llvm-commits]

https://github.com/fhahn updated https://github.com/llvm/llvm-project/pull/109131

>From 903d6d96bf69b6bf8c24e31fc229766fba9a4e6e Mon Sep 17 00:00:00 2001
From: Florian Hahn <flo at fhahn.com>
Date: Wed, 18 Sep 2024 13:38:52 +0100
Subject: [PATCH] [EarlyCSE] Rematerialize alignment assumption.

---
 llvm/lib/Transforms/Scalar/EarlyCSE.cpp             | 13 +++++++++++++
 .../EarlyCSE/materialize-align-assumptions.ll       |  3 +++
 2 files changed, 16 insertions(+)

diff --git a/llvm/lib/Transforms/Scalar/EarlyCSE.cpp b/llvm/lib/Transforms/Scalar/EarlyCSE.cpp
index cf11f5bc885a75..743b333987bc9a 100644
--- a/llvm/lib/Transforms/Scalar/EarlyCSE.cpp
+++ b/llvm/lib/Transforms/Scalar/EarlyCSE.cpp
@@ -31,6 +31,7 @@
 #include "llvm/IR/Constants.h"
 #include "llvm/IR/Dominators.h"
 #include "llvm/IR/Function.h"
+#include "llvm/IR/IRBuilder.h"
 #include "llvm/IR/InstrTypes.h"
 #include "llvm/IR/Instruction.h"
 #include "llvm/IR/Instructions.h"
@@ -1588,6 +1589,18 @@ bool EarlyCSE::processNode(DomTreeNode *Node) {
         if (InVal.IsLoad)
           if (auto *I = dyn_cast<Instruction>(Op))
             combineMetadataForCSE(I, &Inst, false);
+
+        // If the load has align metadata, preserve it via an alignment
+        // assumption. Note that this doesn't use salavageKnowledge, as we need
+        // to create the assumption for the value we replaced the load with.
+        if (auto *AlignMD = Inst.getMetadata(LLVMContext::MD_align)) {
+          auto *A = mdconst::extract<ConstantInt>(AlignMD->getOperand(0));
+          if (Op->getPointerAlignment(SQ.DL).value() % A->getZExtValue() != 0) {
+            IRBuilder B(&Inst);
+            B.CreateAlignmentAssumption(SQ.DL, Op, A);
+          }
+        }
+
         if (!Inst.use_empty())
           Inst.replaceAllUsesWith(Op);
         salvageKnowledge(&Inst, &AC);
diff --git a/llvm/test/Transforms/EarlyCSE/materialize-align-assumptions.ll b/llvm/test/Transforms/EarlyCSE/materialize-align-assumptions.ll
index ea63376957162b..628577b0975071 100644
--- a/llvm/test/Transforms/EarlyCSE/materialize-align-assumptions.ll
+++ b/llvm/test/Transforms/EarlyCSE/materialize-align-assumptions.ll
@@ -10,6 +10,7 @@ define ptr @align_replacement_does_not_have_align_metadata(ptr noalias %p) {
 ; CHECK-NEXT:    call void @foo(ptr [[L_1]])
 ; CHECK-NEXT:    [[GEP:%.*]] = getelementptr i8, ptr [[L_1]], i64 4
 ; CHECK-NEXT:    store ptr [[GEP]], ptr [[P]], align 8
+; CHECK-NEXT:    call void @llvm.assume(i1 true) [ "align"(ptr [[GEP]], i64 4) ]
 ; CHECK-NEXT:    ret ptr [[GEP]]
 ;
   %l.1 = load ptr, ptr %p, align 8
@@ -27,6 +28,7 @@ define ptr @align_replacement_does_not_have_align_metadata2(ptr noalias %p) {
 ; CHECK-NEXT:    [[L_1:%.*]] = load ptr, ptr [[P]], align 8
 ; CHECK-NEXT:    [[GEP:%.*]] = getelementptr i8, ptr [[L_1]], i64 4
 ; CHECK-NEXT:    store ptr [[GEP]], ptr [[P]], align 8
+; CHECK-NEXT:    call void @llvm.assume(i1 true) [ "align"(ptr [[GEP]], i64 4) ]
 ; CHECK-NEXT:    ret ptr [[GEP]]
 ;
   %l.1 = load ptr, ptr %p, align 8
@@ -54,6 +56,7 @@ define ptr @align_replacement_has_smaller_alignment(ptr noalias %p) {
 ; CHECK-SAME: ptr noalias [[P:%.*]]) {
 ; CHECK-NEXT:    [[L_1:%.*]] = load ptr, ptr [[P]], align 8, !align [[META0]]
 ; CHECK-NEXT:    call void @foo(ptr [[L_1]])
+; CHECK-NEXT:    call void @llvm.assume(i1 true) [ "align"(ptr [[L_1]], i64 8) ]
 ; CHECK-NEXT:    ret ptr [[L_1]]
 ;
   %l.1 = load ptr, ptr %p, align 8, !align !0