[EarlyCSE] Rematerialize alignment assumption.

Author: fhahn

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 +1589,16 @@ 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);

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