diff --git a/crates/rustc_codegen_spirv/src/linker/dce.rs b/crates/rustc_codegen_spirv/src/linker/dce.rs
index 472a96b079..40eefa993e 100644
--- a/crates/rustc_codegen_spirv/src/linker/dce.rs
+++ b/crates/rustc_codegen_spirv/src/linker/dce.rs
@@ -7,8 +7,8 @@
 //! *references* a rooted thing is also rooted, not the other way around - but that's the basic
 //! concept.
 
-use rspirv::dr::{Function, Instruction, Module};
-use rspirv::spirv::{Op, Word};
+use rspirv::dr::{Function, Instruction, Module, Operand};
+use rspirv::spirv::{Op, StorageClass, Word};
 use rustc_data_structures::fx::FxHashSet;
 
 pub fn dce(module: &mut Module) {
@@ -36,8 +36,29 @@ fn spread_roots(module: &Module, rooted: &mut FxHashSet<Word>) -> bool {
     }
     for func in &module.functions {
         if rooted.contains(&func.def_id().unwrap()) {
-            for inst in func.all_inst_iter() {
-                any |= root(inst, rooted);
+            // NB (Mobius 2021) - since later insts are much more likely to reference
+            // earlier insts, by reversing the iteration order, we're more likely to root the
+            // entire relevant function at once.
+            // See https://github.com/EmbarkStudios/rust-gpu/pull/691#discussion_r681477091
+            for inst in func
+                .end
+                .iter()
+                .chain(
+                    func.blocks
+                        .iter()
+                        .rev()
+                        .flat_map(|b| b.instructions.iter().rev().chain(b.label.iter())),
+                )
+                .chain(func.parameters.iter().rev())
+                .chain(func.def.iter())
+            {
+                if !instruction_is_pure(inst) {
+                    any |= root(inst, rooted);
+                } else if let Some(id) = inst.result_id {
+                    if rooted.contains(&id) {
+                        any |= root(inst, rooted);
+                    }
+                }
             }
         }
     }
@@ -90,6 +111,13 @@ fn kill_unrooted(module: &mut Module, rooted: &FxHashSet<Word>) {
     module
         .functions
         .retain(|f| is_rooted(f.def.as_ref().unwrap(), rooted));
+    module.functions.iter_mut().for_each(|fun| {
+        fun.blocks.iter_mut().for_each(|block| {
+            block
+                .instructions
+                .retain(|inst| !instruction_is_pure(inst) || is_rooted(inst, rooted));
+        });
+    });
 }
 
 pub fn dce_phi(func: &mut Function) {
@@ -115,3 +143,127 @@ pub fn dce_phi(func: &mut Function) {
             .retain(|inst| inst.class.opcode != Op::Phi || used.contains(&inst.result_id.unwrap()));
     }
 }
+
+fn instruction_is_pure(inst: &Instruction) -> bool {
+    use Op::*;
+    match inst.class.opcode {
+        Nop
+        | Undef
+        | ConstantTrue
+        | ConstantFalse
+        | Constant
+        | ConstantComposite
+        | ConstantSampler
+        | ConstantNull
+        | AccessChain
+        | InBoundsAccessChain
+        | PtrAccessChain
+        | ArrayLength
+        | InBoundsPtrAccessChain
+        | CompositeConstruct
+        | CompositeExtract
+        | CopyObject
+        | Transpose
+        | ConvertFToU
+        | ConvertFToS
+        | ConvertSToF
+        | ConvertUToF
+        | UConvert
+        | SConvert
+        | FConvert
+        | QuantizeToF16
+        | ConvertPtrToU
+        | SatConvertSToU
+        | SatConvertUToS
+        | ConvertUToPtr
+        | PtrCastToGeneric
+        | GenericCastToPtr
+        | GenericCastToPtrExplicit
+        | Bitcast
+        | SNegate
+        | FNegate
+        | IAdd
+        | FAdd
+        | ISub
+        | FSub
+        | IMul
+        | FMul
+        | UDiv
+        | SDiv
+        | FDiv
+        | UMod
+        | SRem
+        | SMod
+        | FRem
+        | FMod
+        | VectorTimesScalar
+        | MatrixTimesScalar
+        | VectorTimesMatrix
+        | MatrixTimesVector
+        | MatrixTimesMatrix
+        | OuterProduct
+        | Dot
+        | IAddCarry
+        | ISubBorrow
+        | UMulExtended
+        | SMulExtended
+        | Any
+        | All
+        | IsNan
+        | IsInf
+        | IsFinite
+        | IsNormal
+        | SignBitSet
+        | LessOrGreater
+        | Ordered
+        | Unordered
+        | LogicalEqual
+        | LogicalNotEqual
+        | LogicalOr
+        | LogicalAnd
+        | LogicalNot
+        | Select
+        | IEqual
+        | INotEqual
+        | UGreaterThan
+        | SGreaterThan
+        | UGreaterThanEqual
+        | SGreaterThanEqual
+        | ULessThan
+        | SLessThan
+        | ULessThanEqual
+        | SLessThanEqual
+        | FOrdEqual
+        | FUnordEqual
+        | FOrdNotEqual
+        | FUnordNotEqual
+        | FOrdLessThan
+        | FUnordLessThan
+        | FOrdGreaterThan
+        | FUnordGreaterThan
+        | FOrdLessThanEqual
+        | FUnordLessThanEqual
+        | FOrdGreaterThanEqual
+        | FUnordGreaterThanEqual
+        | ShiftRightLogical
+        | ShiftRightArithmetic
+        | ShiftLeftLogical
+        | BitwiseOr
+        | BitwiseXor
+        | BitwiseAnd
+        | Not
+        | BitFieldInsert
+        | BitFieldSExtract
+        | BitFieldUExtract
+        | BitReverse
+        | BitCount
+        | Phi
+        | SizeOf
+        | CopyLogical
+        | PtrEqual
+        | PtrNotEqual
+        | PtrDiff => true,
+        Variable => inst.operands.get(0) == Some(&Operand::StorageClass(StorageClass::Function)),
+        _ => false,
+    }
+}
diff --git a/tests/ui/dis/index_user_dst.stderr b/tests/ui/dis/index_user_dst.stderr
index 2cc20726c0..2335846a19 100644
--- a/tests/ui/dis/index_user_dst.stderr
+++ b/tests/ui/dis/index_user_dst.stderr
@@ -5,33 +5,32 @@ OpLine %5 7 12
 %10 = OpArrayLength  %11  %8 0
 OpLine %5 7 0
 %12 = OpCompositeInsert  %13  %6 %14 0
-%15 = OpCompositeConstruct  %13  %6 %10
 OpLine %5 8 21
-%16 = OpULessThan  %17  %9 %10
+%15 = OpULessThan  %16  %9 %10
 OpLine %5 8 21
-OpSelectionMerge %18 None
-OpBranchConditional %16 %19 %20
-%19 = OpLabel
+OpSelectionMerge %17 None
+OpBranchConditional %15 %18 %19
+%18 = OpLabel
 OpLine %5 8 21
-%21 = OpInBoundsAccessChain  %22  %6 %9
-%23 = OpLoad  %24  %21
+%20 = OpInBoundsAccessChain  %21  %6 %9
+%22 = OpLoad  %23  %20
 OpLine %5 10 1
 OpReturn
-%20 = OpLabel
+%19 = OpLabel
 OpLine %5 8 21
+OpBranch %24
+%24 = OpLabel
 OpBranch %25
 %25 = OpLabel
-OpBranch %26
-%26 = OpLabel
-%27 = OpPhi  %17  %28 %25 %28 %29
-OpLoopMerge %30 %29 None
-OpBranchConditional %27 %31 %30
-%31 = OpLabel
-OpBranch %29
-%29 = OpLabel
-OpBranch %26
+%26 = OpPhi  %16  %27 %24 %27 %28
+OpLoopMerge %29 %28 None
+OpBranchConditional %26 %30 %29
 %30 = OpLabel
+OpBranch %28
+%28 = OpLabel
+OpBranch %25
+%29 = OpLabel
 OpUnreachable
-%18 = OpLabel
+%17 = OpLabel
 OpUnreachable
 OpFunctionEnd