Fix crash when heap subscript result is discarded

docs/ReleaseNotes.md

@@ -30,6 +30,9 @@ line upon naming the release. Refer to previous for appropriate section names.
 - SPIR-V: Fixed an invalid `OpSelect` being generated when optimizing for
   SPIR-V 1.3 and earlier
   [#8603](https://github.com/microsoft/DirectXShaderCompiler/issues/8603).
+- Fix a crash generating DXIL from sources containing a dynamic resource heap
+  access that was discarded. Identified during development of SPIR-V support for
+  [descriptor heaps](https://github.com/microsoft/DirectXShaderCompiler/pull/8517#discussion_r3752113078).
 
 #### HLSL Language
 

tools/clang/lib/CodeGen/CGHLSLMSFinishCodeGen.cpp

@@ -12,6 +12,7 @@
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/ADT/StringRef.h"
 #include "llvm/Analysis/DxilValueCache.h"
+#include "llvm/Analysis/ValueTracking.h"
 #include "llvm/IR/CFG.h"
 #include "llvm/IR/DerivedTypes.h"
 #include "llvm/IR/Function.h"
@@ -330,6 +331,13 @@ void LowerGetResourceFromHeap(
     for (auto uit = resPtr->user_begin(); uit != resPtr->user_end();) {
       User *U = *(uit++);
       BitCastInst *BCI = cast<BitCastInst>(U);
+      // Ignore uses of the resource which are just bitcasts to i8* for lifetime
+      // markers. These will get cleaned up in later legalization.
+      if (BCI->getType()->getPointerElementType()->isIntegerTy(8)) {
+        DXASSERT(onlyUsedByLifetimeMarkers(BCI),
+                 "otherwise, unexpected use of i8* cast of resource ptr");
+        continue;
+      }
       DXASSERT(
           dxilutil::IsHLSLResourceType(
               BCI->getType()->getPointerElementType()) ||
@@ -348,7 +356,10 @@ void LowerGetResourceFromHeap(
       }
       BCI->eraseFromParent();
     }
-    resPtr->eraseFromParent();
+    // Only erase the resource if it has no remaining uses. The correct fix here
+    // is to just not generate these resources, but that is a larger change.
+    if (resPtr->use_empty())
+      resPtr->eraseFromParent();
   }
 }
 

tools/clang/test/CodeGenDXIL/discarded_dynamic_res.hlsl

@@ -0,0 +1,48 @@
+// RUN: %dxc -T cs_6_6 -fcgl %s | FileCheck %s
+// RUN: %dxc -T cs_6_6 -Od -fcgl %s | FileCheck --check-prefix=NO_LIFETIMES %s
+
+// Regression test: indexing into a resource heap (or sampler heap) as a
+// discarded-value expression (i.e. its result is never read) used to crash
+// the compiler. Clang still emits an alloca plus lifetime markers for the
+// implicit temporary in that case, and CGHLSLMSHelper::LowerGetResourceFromHeap
+// only expected the resource pointer to be used through a resource-typed
+// bitcast that is loaded from; it did not expect the i8* bitcast used by
+// llvm.lifetime.start/llvm.lifetime.end, which triggered an invalid cast<>
+// (see FinishIntrinsics -> LowerGetResourceFromHeap).
+
+// When `-Od` is passed lifetime markers are implicitly disabled, so the
+// remainder of the IR changes are irrelevant.
+// NO_LIFETIMES: define void @main
+
+// CHECK: [[ResourceX:%.*]] = alloca %struct..Resource
+// CHECK: [[SamplerX:%.*]] = alloca %struct..Sampler
+// CHECK: [[ResourceY:%.*]] = alloca %struct..Resource
+// CHECK: [[SamplerY:%.*]] = alloca %struct..Sampler
+
+[numthreads(1, 1, 1)]
+void main(uint3 tid : SV_DispatchThreadID) {
+  // CHECK: [[Start:%.*]] = bitcast %struct..Resource* [[ResourceX]] to i8*
+  // CHECK-NEXT:  call void @llvm.lifetime.start(i64 4, i8* [[Start]])
+  // CHECK: [[End:%.*]] = bitcast %struct..Resource* [[ResourceX]] to i8*
+  // CHECK-NEXT:  call void @llvm.lifetime.end(i64 4, i8* [[End]])
+  ResourceDescriptorHeap[tid.x];
+
+  // CHECK: [[Start:%.*]] = bitcast %struct..Sampler* [[SamplerX]] to i8*
+  // CHECK-NEXT:  call void @llvm.lifetime.start(i64 4, i8* [[Start]])
+  // CHECK: [[End:%.*]] = bitcast %struct..Sampler* [[SamplerX]] to i8*
+  // CHECK-NEXT:  call void @llvm.lifetime.end(i64 4, i8* [[End]])
+  SamplerDescriptorHeap[tid.x];
+
+
+  // CHECK: [[Start:%.*]] = bitcast %struct..Resource* [[ResourceY]] to i8*
+  // CHECK-NEXT:  call void @llvm.lifetime.start(i64 4, i8* [[Start]])
+  // CHECK: [[End:%.*]] = bitcast %struct..Resource* [[ResourceY]] to i8*
+  // CHECK-NEXT:  call void @llvm.lifetime.end(i64 4, i8* [[End]])
+  (void)ResourceDescriptorHeap[tid.y];
+
+  // CHECK: [[Start:%.*]] = bitcast %struct..Sampler* [[SamplerY]] to i8*
+  // CHECK-NEXT:  call void @llvm.lifetime.start(i64 4, i8* [[Start]])
+  // CHECK: [[End:%.*]] = bitcast %struct..Sampler* [[SamplerY]] to i8*
+  // CHECK-NEXT:  call void @llvm.lifetime.end(i64 4, i8* [[End]])
+  (void)SamplerDescriptorHeap[tid.y];
+}