Check if const is zero before getting components. (#5217)

* Check if const is zero before getting components.

Two folding rules try to cast a constant to a MatrixConstant before
checking if it is a Null constant. This leads to the null pointer being
dereferneced. The solution is to move the check for zero earlier.

Fixes https://github.com/microsoft/DirectXShaderCompiler/issues/5063

source/opt/const_folding_rules.cpp

@@ -376,13 +376,7 @@ ConstantFoldingRule FoldVectorTimesMatrix() {
     assert(c1->type()->AsVector()->element_type() == element_type &&
            c2->type()->AsMatrix()->element_type() == vector_type);
 
-    // Get a float vector that is the result of vector-times-matrix.
-    std::vector<const analysis::Constant*> c1_components =
-        c1->GetVectorComponents(const_mgr);
-    std::vector<const analysis::Constant*> c2_components =
-        c2->AsMatrixConstant()->GetComponents();
     uint32_t resultVectorSize = result_type->AsVector()->element_count();
-
     std::vector<uint32_t> ids;
 
     if ((c1 && c1->IsZero()) || (c2 && c2->IsZero())) {
@@ -395,6 +389,12 @@ ConstantFoldingRule FoldVectorTimesMatrix() {
       return const_mgr->GetConstant(vector_type, ids);
     }
 
+    // Get a float vector that is the result of vector-times-matrix.
+    std::vector<const analysis::Constant*> c1_components =
+        c1->GetVectorComponents(const_mgr);
+    std::vector<const analysis::Constant*> c2_components =
+        c2->AsMatrixConstant()->GetComponents();
+
     if (float_type->width() == 32) {
       for (uint32_t i = 0; i < resultVectorSize; ++i) {
         float result_scalar = 0.0f;
@@ -472,13 +472,7 @@ ConstantFoldingRule FoldMatrixTimesVector() {
     assert(c1->type()->AsMatrix()->element_type() == vector_type);
     assert(c2->type()->AsVector()->element_type() == element_type);
 
-    // Get a float vector that is the result of matrix-times-vector.
-    std::vector<const analysis::Constant*> c1_components =
-        c1->AsMatrixConstant()->GetComponents();
-    std::vector<const analysis::Constant*> c2_components =
-        c2->GetVectorComponents(const_mgr);
     uint32_t resultVectorSize = result_type->AsVector()->element_count();
-
     std::vector<uint32_t> ids;
 
     if ((c1 && c1->IsZero()) || (c2 && c2->IsZero())) {
@@ -491,6 +485,12 @@ ConstantFoldingRule FoldMatrixTimesVector() {
       return const_mgr->GetConstant(vector_type, ids);
     }
 
+    // Get a float vector that is the result of matrix-times-vector.
+    std::vector<const analysis::Constant*> c1_components =
+        c1->AsMatrixConstant()->GetComponents();
+    std::vector<const analysis::Constant*> c2_components =
+        c2->GetVectorComponents(const_mgr);
+
     if (float_type->width() == 32) {
       for (uint32_t i = 0; i < resultVectorSize; ++i) {
         float result_scalar = 0.0f;

test/opt/fold_test.cpp

@@ -290,7 +290,7 @@ OpName %main "main"
 %v4float_1_1_1_1 = OpConstantComposite %v4float %float_1 %float_1 %float_1 %float_1
 %v4float_1_2_3_4 = OpConstantComposite %v4float %float_1 %float_2 %float_3 %float_4
 %v4float_null = OpConstantNull %v4float
-%mat4v4float_null = OpConstantComposite %mat4v4float %v4float_null %v4float_null %v4float_null %v4float_null
+%mat4v4float_null = OpConstantNull %mat4v4float
 %mat4v4float_1_2_3_4 = OpConstantComposite %mat4v4float %v4float_1_2_3_4 %v4float_1_2_3_4 %v4float_1_2_3_4 %v4float_1_2_3_4
 %mat4v4float_1_2_3_4_null = OpConstantComposite %mat4v4float %v4float_1_2_3_4 %v4float_null %v4float_1_2_3_4 %v4float_null
 %107 = OpConstantComposite %v4double %double_0 %double_0 %double_0 %double_0
@@ -301,7 +301,7 @@ OpName %main "main"
 %v4double_1_2_3_4 = OpConstantComposite %v4double %double_1 %double_2 %double_3 %double_4
 %v4double_1_1_1_0p5 = OpConstantComposite %v4double %double_1 %double_1 %double_1 %double_0p5
 %v4double_null = OpConstantNull %v4double
-%mat4v4double_null = OpConstantComposite %mat4v4double %v4double_null %v4double_null %v4double_null %v4double_null
+%mat4v4double_null = OpConstantNull %mat4v4double
 %mat4v4double_1_2_3_4 = OpConstantComposite %mat4v4double %v4double_1_2_3_4 %v4double_1_2_3_4 %v4double_1_2_3_4 %v4double_1_2_3_4
 %mat4v4double_1_2_3_4_null = OpConstantComposite %mat4v4double %v4double_1_2_3_4 %v4double_null %v4double_1_2_3_4 %v4double_null
 %v4float_n1_2_1_3 = OpConstantComposite %v4float %float_n1 %float_2 %float_1 %float_3