[DAGCombine] Generalize Dead Store to overlapping stores.

[niravd]

Summary

Remove stores that are immediately overwritten by larger
stores.

[rnk]

lgtm, nice

llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp

if (Ld->getBasePtr() == Ptr && ST->getMemoryVT() == Ld->getMemoryVT() &&
         Chain.reachesChainWithoutSideEffects(SDValue(Ld, 1))) {
       // The store is dead, remove it.
       return Chain;
     }
   }
 
   if (StoreSDNode *ST1 = dyn_cast<StoreSDNode>(Chain)) {
     if (ST->isUnindexed() && !ST->isVolatile() && ST1->isUnindexed() &&
-        !ST1->isVolatile() && ST1->getBasePtr() == Ptr &&
+        !ST1->isVolatile()) {
+      if (ST1->getBasePtr() == Ptr && ST1->getValue() == Value &&
           ST->getMemoryVT() == ST1->getMemoryVT()) {
-      // If this is a store followed by a store with the same value to the same
-      // location, then the store is dead/noop.
-      if (ST1->getValue() == Value) {
-        // The store is dead, remove it.
+        // If this is a store followed by a store with the same value to the
+        // same location, then the store is dead/noop.
         return Chain;
       }
 
-      // If this is a store who's preceeding store to the same location
-      // and no one other node is chained to that store we can effectively
-      // drop the store. Do not remove stores to undef as they may be used as
-      // data sinks.
       if (OptLevel != CodeGenOpt::None && ST1->hasOneUse() &&
           !ST1->getBasePtr().isUndef()) {
-        // ST1 is fully overwritten and can be elided. Combine with it's chain
-        // value.
+        const BaseIndexOffset STBase = BaseIndexOffset::match(ST, DAG);
+        const BaseIndexOffset ChainBase = BaseIndexOffset::match(ST1, DAG);
+        unsigned STByteSize = ST->getMemoryVT().getSizeInBits() / 8;
+        unsigned ChainByteSize = ST1->getMemoryVT().getSizeInBits() / 8;
+        // If this is a store who's preceeding store to a subset of the current
+        // location and no one other node is chained to that store we can
+        // effectively drop the store. Do not remove stores to undef as they may
+        // be used as data sinks.
+        if (STBase.contains(STByteSize, ChainBase, ChainByteSize, DAG)) {
           CombineTo(ST1, ST1->getChain());
           return SDValue();
         }
       }
     }
+  }
 
   // If this is an FP_ROUND or TRUNC followed by a store, fold this into a
   // truncating store.  We can do this even if this is already a truncstore.
   if ((Value.getOpcode() == ISD::FP_ROUND || Value.getOpcode() == ISD::TRUNCATE)
       && Value.getNode()->hasOneUse() && ST->isUnindexed() &&
       TLI.isTruncStoreLegal(Value.getOperand(0).getValueType(),
                             ST->getMemoryVT())) {
     return DAG.getTruncStore(Chain, SDLoc(N), Value.getOperand(0),

llvm/test/CodeGen/AArch64/ldst-paired-aliasing.ll

 ; RUN: llc -mcpu cortex-a53 < %s | FileCheck %s
 target datalayout = "e-m:e-i64:64-i128:128-n8:16:32:64-S128"
 target triple = "aarch64--linux-gnu"
 
 declare void @f(i8*, i8*)
 declare void @f2(i8*, i8*)
 declare void @_Z5setupv()
 declare void @llvm.memset.p0i8.i64(i8* nocapture, i8, i64, i1) #3
 
 define i32 @main() local_unnamed_addr #1 {
 ; Make sure the stores happen in the correct order (the exact instructions could change).
 ; CHECK-LABEL: main:
-; CHECK: stp xzr, xzr, [sp, #72]
+
+; CHECK: str xzr, [sp, #80]
 ; CHECK: str w9, [sp, #80]
-; CHECK: str q0, [sp, #48]
+; CHECK: stp q0, q0, [sp, #48]
 ; CHECK: ldr w8, [sp, #48]
-; CHECK: str q0, [sp, #64]
 
 for.body.lr.ph.i.i.i.i.i.i63:
   %b1 = alloca [10 x i32], align 16
   %x0 = bitcast [10 x i32]* %b1 to i8*
   %b2 = alloca [10 x i32], align 16
   %x1 = bitcast [10 x i32]* %b2 to i8*
   tail call void @_Z5setupv()
   %x2 = getelementptr inbounds [10 x i32], [10 x i32]* %b1, i64 0, i64 6

llvm/test/CodeGen/X86/pr40631_deadstore_elision.ll

 ; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py
 ; RUN: llc -o - %s -mtriple=x86_64-unknown-linux-gnu | FileCheck %s
 
 %struct.sk_buff = type { i8* }
 %struct.xt_action_param = type { i8*, i8*, i8*, i32, i32, i8 }
 
 define i32 @ipt_do_table(%struct.sk_buff* noalias nocapture readonly) {
 ; CHECK-LABEL: ipt_do_table:
 ; CHECK:       # %bb.0:
 ; CHECK-NEXT:    subq $40, %rsp
 ; CHECK-NEXT:    .cfi_def_cfa_offset 48
 ; CHECK-NEXT:    movq (%rdi), %rax
 ; CHECK-NEXT:    xorps %xmm0, %xmm0
-; CHECK-NEXT:    movaps %xmm0, (%rsp)
 ; CHECK-NEXT:    movaps %xmm0, {{[0-9]+}}(%rsp)
 ; CHECK-NEXT:    movq $0, {{[0-9]+}}(%rsp)
 ; CHECK-NEXT:    movaps {{.*#+}} xmm0 = [12297829382473034410,12297829382473034410]
 ; CHECK-NEXT:    movaps %xmm0, (%rsp)
 ; CHECK-NEXT:    movabsq $-6148914691236517206, %rcx # imm = 0xAAAAAAAAAAAAAAAA
 ; CHECK-NEXT:    movq %rcx, {{[0-9]+}}(%rsp)
 ; CHECK-NEXT:    movq %rcx, {{[0-9]+}}(%rsp)
 ; CHECK-NEXT:    movb $-86, {{[0-9]+}}(%rsp)