Make asm constant substitution scope- and flow-aware (bug) by axic · Pull Request #546 · argotorg/solcore

axic · 2026-08-04T20:03:20Z

evalStmt (MastAsm yul) inlined every literal-valued binding in scope into
the asm block with a single, uniform substitution map. That map was neither
scope- nor flow-aware, so it produced wrong code in two ways:

Flow: after an in-block x := 5, later reads of x were still replaced
by the stale pre-block literal (y := add(x, 1) folded to add(0, 1)).
Shadowing: an asm-local let i (including a for-loop counter, plain
let, and function parameters/returns) was overwritten by the outer
literal, e.g. for { let i := 0 } lt(i, 3) { ... } became lt(0, 3) — an
infinite loop.

substYulBlock now threads the substitution map through the statement
sequence: an assignment drops the reassigned name, a let/parameter/return
drops the shadowed name for the rest of its scope, and assignments that escape
a nested if/for/switch/block invalidate the outer literal afterwards.
Yul function bodies have an isolated scope, so they neither leak shadowing
outward nor let inner assignments invalidate outer literals.

Adds direct unit tests for substYulBlock in YulEvalTests covering the flow
case, the shadowing cases (plain let, for counter, loop-assigned variable,
function params/returns), let-initialiser scope, and nested-block escape.

Before the fix the test case returns this:

Expected 0000000000000000000000000000000000000000000000000000000000000003 but got 0000000000000000000000000000000000000000000000000000000000000001 (comment: asmLoopFlow()(uint256) -> 3 (accumulate s across 3 iterations))

mbenke

Good catch. Comptime may need a systematic review, but let's merge this for now. Just two small suggestions re tests.

mbenke · 2026-08-05T09:32:46Z

+    // Flow-sensitivity: after `x := 5` the read of `x` in `add(x, 1)` must see
+    // the new value 5, not the pre-block literal 0.  The buggy substitution
+    // folded the second statement to `add(0, 1)` and returned 1; the correct
+    // result is 6.


Actually this seems to return 6 on main as well:

$ ./runsol.sh tmp/asm_subst.solc --runtime-calldata 'asmFlow()(uint)' Processing: tmp/asm_subst.solc ... Execution successful Decoded output: 6 Gas used: 569

I think this was overly eager claude, the loop one is the buggy one.

So what do you plan to do? Fix the test or remove it? I suggest a possible fix below

mbenke · 2026-08-05T09:32:49Z

+        let x : word = 0;
+        let y : word = 0;
+        assembly {
+          x := 5


This may make the test fail without the fix:

Suggested change

x := 5

x := add(5, sload(1234))

mbenke · 2026-08-05T09:58:01Z

+      -- The reported infinite-loop case: the for-counter is `let`-bound in the
+      -- pre-block, so neither the condition nor the body may inline the outer 0.
+      testCase "a for-loop counter shadowed by its pre-let is not inlined" $


Both the name and the comment above it attribute the result to the pre-let, but it seems that what actually keeps i out of the map is the post-block:

[yAssign "i" (yCall "add" [yIdent "i", yNum 1])] -- post

which gives escapingAssignsYulBlock post = {i}. The pre-let contributes nothing.

Suggested change

-- The reported infinite-loop case: the for-counter is `let`-bound in the

-- pre-block, so neither the condition nor the body may inline the outer 0.

testCase "a for-loop counter shadowed by its pre-let is not inlined" $

-- The reported infinite-loop case. `i` is assigned in the post-block, so it

-- is not loop-invariant and must not be inlined anywhere in the loop. The

-- pre-let's own shadowing is exercised separately, by the loop-invariant case

-- below

testCase "a for-loop counter assigned in the post-block is not inlined" $

Complements the substYulBlock unit tests with an executable contract that runs through the full pipeline (sol-core → yule → solc → testrunner) and asserts real EVM return values, so the miscompilation is caught end to end: * asmFlow() — `x := 5; y := add(x, 1)` returns 6 (buggy: 1). * asmLoopFlow() — a loop body accumulating an outer `s` returns 3 (buggy: the pre-loop literal `s == 0` is inlined into the body, so `s` is reset to 1 each iteration and the function returns 1). Registered in run_contests.sh (executes and checks return data) and in the tasty `dispatches` group in Cases.hs (compile coverage under `cabal test`). Co-Authored-By: Alex Beregszaszi <alex@rtfs.hu>

`evalStmt (MastAsm yul)` inlined every literal-valued binding in scope into the asm block with a single, uniform substitution map. That map was neither scope- nor flow-aware, so it produced wrong code in two ways: * Flow: after an in-block `x := 5`, later reads of `x` were still replaced by the stale pre-block literal (`y := add(x, 1)` folded to `add(0, 1)`). * Shadowing: an asm-local `let i` (including a `for`-loop counter, plain `let`, and function parameters/returns) was overwritten by the outer literal, e.g. `for { let i := 0 } lt(i, 3) { ... }` became `lt(0, 3)` — an infinite loop. `substYulBlock` now threads the substitution map through the statement sequence: an assignment drops the reassigned name, a `let`/parameter/return drops the shadowed name for the rest of its scope, and assignments that escape a nested `if`/`for`/`switch`/block invalidate the outer literal afterwards. Yul function bodies have an isolated scope, so they neither leak shadowing outward nor let inner assignments invalidate outer literals. Adds direct unit tests for `substYulBlock` in YulEvalTests covering the flow case, the shadowing cases (plain `let`, `for` counter, loop-assigned variable, function params/returns), let-initialiser scope, and nested-block escape. Co-Authored-By: Alex Beregszaszi <alex@rtfs.hu>

mbenke · 2026-08-06T08:47:04Z

+        let x : word = 0;
+        let y : word = 0;
+        assembly {
+          x := 5


x := add(5, sload(1234))

mbenke · 2026-08-06T08:47:08Z

+    // Flow-sensitivity: after `x := 5` the read of `x` in `add(x, 1)` must see
+    // the new value 5, not the pre-block literal 0.  The buggy substitution
+    // folded the second statement to `add(0, 1)` and returned 1; the correct
+    // result is 6.


So what do you plan to do? Fix the test or remove it? I suggest a possible fix below

axic force-pushed the yul-subst-block-bug branch from 21aa68c to 494b7a0 Compare August 4, 2026 20:06

mbenke approved these changes Aug 5, 2026

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mbenke mentioned this pull request Aug 5, 2026

Assigning to a comptime variable generates incorrect Hull (and fails in yule) #549

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claude and others added 2 commits August 5, 2026 16:02

axic force-pushed the yul-subst-block-bug branch from 494b7a0 to 229c32e Compare August 5, 2026 16:02

mbenke reviewed Aug 6, 2026

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Make asm constant substitution scope- and flow-aware (bug) - #546

Make asm constant substitution scope- and flow-aware (bug)#546
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-      -- The reported infinite-loop case: the for-counter is `let`-bound in the
-      -- pre-block, so neither the condition nor the body may inline the outer 0.
-      testCase "a for-loop counter shadowed by its pre-let is not inlined" $
+-- The reported infinite-loop case.  `i` is assigned in the post-block, so it
+-- is not loop-invariant and must not be inlined anywhere in the loop.  The
+-- pre-let's own shadowing is exercised separately, by the loop-invariant case
+-- below
+testCase "a for-loop counter assigned in the post-block is not inlined" $

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axic commented Aug 4, 2026 •

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