Fix stack compression in stack layout generator.

Author: ekpyron

libyul/backends/evm/StackLayoutGenerator.cpp

@@ -336,7 +336,9 @@ Stack StackLayoutGenerator::propagateStackThroughOperation(Stack _exitStack, CFG
 			stack.pop_back();
 		else if (auto offset = util::findOffset(stack | ranges::views::reverse | ranges::views::drop(1), stack.back()))
 		{
-			if (*offset + 2 < m_reachableStackDepth)
+			// *offset == n requires, after popping the topmost slot, a DUP<n+1> to recover it.
+			// So if *offset + 1 is within stack reach, we can pop.
+			if (*offset + 1 <= m_reachableStackDepth)
 				stack.pop_back();
 			else
 				break;

test/libsolidity/semanticTests/externalContracts/base64.sol

@@ -33,8 +33,8 @@ contract test {
 // EVMVersion: >=constantinople
 // ----
 // constructor()
-// gas irOptimized: 79076
-// gas irOptimized code: 322000
+// gas irOptimized: 78830
+// gas irOptimized code: 319000
 // gas legacy: 102214
 // gas legacy code: 629800
 // gas legacyOptimized: 87926
@@ -58,6 +58,6 @@ contract test {
 // gas legacy: 1554038
 // gas legacyOptimized: 1132031
 // encode_no_asm_large()
-// gas irOptimized: 3512081
+// gas irOptimized: 3497081
 // gas legacy: 4600082
 // gas legacyOptimized: 2813075

test/libsolidity/semanticTests/externalContracts/deposit_contract.sol

@@ -176,8 +176,8 @@ contract DepositContract is IDepositContract, ERC165 {
 }
 // ----
 // constructor()
-// gas irOptimized: 809570
-// gas irOptimized code: 558000
+// gas irOptimized: 809356
+// gas irOptimized code: 555400
 // gas legacy: 920228
 // gas legacy code: 1438800
 // gas legacyOptimized: 848699

test/libsolidity/semanticTests/externalContracts/snark.sol

@@ -297,6 +297,6 @@ contract Test {
 // gas legacyOptimized: 276313
 // verifyTx() -> true
 // ~ emit Verified(string): 0x20, 0x16, "Successfully verified."
-// gas irOptimized: 818076
+// gas irOptimized: 817995
 // gas legacy: 904397
 // gas legacyOptimized: 816770

test/libsolidity/semanticTests/types/mapping/copy_from_mapping_to_mapping.sol

@@ -29,6 +29,6 @@ contract C {
 }
 // ----
 // f() -> 0x20, 7, 8, 9, 0xa0, 13, 2, 0x40, 0xa0, 2, 3, 4, 2, 3, 4
-// gas irOptimized: 197102
+// gas irOptimized: 197092
 // gas legacy: 199887
 // gas legacyOptimized: 196845