Solidity breaks on skip when compiled with viaIR

[CodeSandwich]

Component

Forge

Have you ensured that all of these are up to date?

• Foundry
• Foundryup

What version of Foundry are you on?

forge 0.2.0 (3a462eb 2022-09-22T00:05:16.281308046Z)

What command(s) is the bug in?

forge test

Operating System

Linux

Describe the bug

I have a trivial test:

import {Test} from "forge-std/Test.sol";

contract SkipTest is Test {
    function testSkip() public {
        uint256 value = block.timestamp;
        emit log_uint(value);
        skip(1);
        emit log_uint(value);
    }
}

When compiling with via_ir = true, the output is:

[PASS] testSkip() (gas: 5134)
Logs:
  1
  2

Solidity seems to make an assumption that block.timestamp never changes during a call, so it doesn't make sense to actually store it in a variable. It makes perfect sense to optimize away the stack storage of a constant, but it breaks test EVMs with changeable timestamps.

I'm not sure if this is solveable in Foundry, it may be a Solidity issue. OTOH it's overall a good optimization, it breaks only with Foundry's test framework architecture, why should every DeFi user in the world pay gas for that?

[CodeSandwich]

A workaround I've found for this is to force the compiler to perform a seemingly non-no-op operation, e.g.

uint256 value = uint248(block.timestamp);

But NOT trivial things like + 1 - 1, the compiler is too smart for that. But I guess that it'll become harder in the future.

[mds1]

See #1373 for more info + potential solutions, glad you found another workaround

[CodeSandwich]

Thank you! Closing it then as a duplicate.