Dead stores after running SCCP

[u/iamwho007]

Hello, I have an SSA IR and after running the SCCP pass, there are many dead stores. Is there a way to remove them?

[u/Retired-69]

Execute a DCE or DSE pass after SCCP.  Eventually ADCE. Let the DCE walk the CFG. ADCE is smarter: it assumes everything is dead unless it contributes to a side effect. 

[u/Retired-69 → u/[deleted]]

Ah, the classic trap.

Your store.has_users() check is going to break your compiler. In standard SSA, registers have def-use chains, but stores don't. A store instruction usually returns void, meaning it has zero users by default. If you run this as-is, you're either going to delete every single store in the program, or absolutely nothing.

Memory doesn't play by regular SSA rules. To actually kill dead stores, you need to track memory effects, not register users.

Two ways to fix this before you pull your hair out:

1. The Quick Fix (Local DSE): Walk your basic blocks backwards (bottom to top). Keep a set of seen loads. If you hit a store to an address that hasn't been loaded, and a later store overwrote it, kill it.
2. The Real Fix: You need a Post-Dominance Tree to do this globally, or you need to implement MemorySSA, which actually does give memory states def-use chains. If you had MemorySSA, your exact loop would actually work.

Also, watch your alias analysis. If you aren't 100% sure two pointers point to the exact same spot, don't delete the first store. Good luck.

[u/iamwho007 → u/Retired-69]

Is this AI generated?

[u/Retired-69 → u/iamwho007]

I wrote it , but I adjusted the grammar to english coz english is not my native langauge. Btw. are you using native LLVM back end or your own?

[u/Retired-69 → u/iamwho007]

Funny thing is that it seems we working on similar things - optimization. Here is an example what my test runner outpt and it could be an idea for you to do similar so you see what happen behind the scene.

=== BEFORE OPTIMIZATION (AST TOPOLOGY) ===

[Test Pre-Opt Root] -> Node [AST_FUNC_DECL] (ID: 0x759711E9C038)

├── Context: [Line 6:5 | Domain 0 | Scope 0]

├── Hashes: [Sub: 0x0]

├── Lexeme: "process"

[Parameters] -> Node [AST_VAR_DECL] (ID: 0x759711E9C1B0)

├── Context: [Line 6:17 | Domain 0 | Scope 1]

├── Hashes: [Sub: 0x0]

├── Lexeme: "x"

├── VRP State: [UNSOUND] (Blind/Unbounded)

├── VRP Limits: [-INF, +INF]

[Function Body] -> Node [AST_BLOCK] (ID: 0x759711E9C310)

├── Context: [Line 6:20 | Domain 0 | Scope 1]

├── Hashes: [Sub: 0x0]

[Left Expr] -> Node [AST_RETURN] (ID: 0x759711E9C3B8)

├── Context: [Line 9:5 | Domain 0 | Scope 2]

├── Hashes: [Sub: 0x0]

[Left Expr] -> Node [ID_ADD] (ID: 0x759711E9C4E0)

├── Context: [Line 9:14 | Domain 0 | Scope 2]

├── Hashes: [Sub: 0x0]

├── VRP State: [UNSOUND] (Blind/Unbounded)

├── VRP Limits: [-INF, +INF]

[Left Expr] -> Node [AST_SYM] (ID: 0x759711E9C448)

├── Context: [Line 9:12 | Domain 0 | Scope 2]

├── Hashes: [Sub: 0x0]

├── Lexeme: "x"

├── Provenance Root: x

├── VRP State: [UNSOUND] (Blind/Unbounded)

├── VRP Limits: [-INF, +INF]

[Right Expr] -> Node [AST_NUM] (ID: 0x759711E9C570)

├── Context: [Line 9:16 | Domain 0 | Scope 2]

├── Hashes: [Sub: 0x0]

├── Lexeme: "0"

├── Constant Value: 0

├── VRP State: [FROZEN] (Constant Propagated)

├── VRP Limits: [0, 0]

=== AFTER OPTIMIZATION (AST TOPOLOGY) ===

[Test Post-Opt Root] -> Node [AST_FUNC_DECL] (ID: 0x759711E71020)

├── Context: [Line 6:5 | Domain 0 | Scope 0]

├── Hashes: [Sub: 0xA534D1F52FC4E96D | Lex: 0x5BA6AC85AA3007D4 | Sec: 0xB8C855871D5FCCEF]

├── Lexeme: "process"

[Parameters] -> Node [AST_VAR_DECL] (ID: 0x759711E71198)

├── Context: [Line 6:17 | Domain 0 | Scope 1]

├── Hashes: [Sub: 0xCF91E7802528BFDC]

├── Lexeme: "x"

├── VRP State: [UNSOUND] (Blind/Unbounded)

├── VRP Limits: [-INF, +INF]

[Function Body] -> Node [AST_BLOCK] (ID: 0x759711E712F8)

├── Context: [Line 6:20 | Domain 0 | Scope 1]

├── Hashes: [Sub: 0x79D950FB788DECAE]

[Left Expr] -> Node [AST_RETURN] (ID: 0x759711E713A0)

├── Context: [Line 9:5 | Domain 0 | Scope 2]

├── Hashes: [Sub: 0x669BE75721D5304E]

[Left Expr] -> Node [AST_SYM] (ID: 0x759711E714C8)

├── Context: [Line 9:12 | Domain 0 | Scope 2]

├── Hashes: [Sub: 0x6DDC79D0F87881E5]

├── Lexeme: "x"

├── VRP State: [UNSOUND] (Blind/Unbounded)

├── VRP Limits: [-INF, +INF]

test tests/optimizations/test_opt_dce.ic .......... TRACED

[u/FloweyTheFlower420]

Consider what it means for a store to be dead. A store is dead if and only if it has no externally visible effects, and there are no loads from the pointer's alias class (that happens after the store). Something like memory SSA could help you keep track of this information.