SSA: Remove some shuffler logic

This piece of logic was swapping top to the tail region if case the top
is not in its right place and it wants to be in tail anyway.

I don't have an explanation why this is useful, but it seems to improve
code generation on most external benchmarks.

Author: blishko

libyul/backends/evm/ssa/StackShuffler.h

@@ -446,23 +446,6 @@ class StackShuffler
 						_stack.swap(offset);
 						return {ShuffleHelperResult::Status::StackModified};
 					}
-
-				// try swapping top with a tail slot that has what we need at top
-				for (StackOffset tailOffset: _state.stackTailRange())
-					if (
-						_stack.isValidSwapTarget(tailOffset) &&
-						_state.isArgsCompatible(tailOffset, stackTop) &&
-						(!_state.requiredInTail(_stack[tailOffset]) || _state.countInTail(_stack[tailOffset]) > 1) &&
-						// current top can safely go to tail (not needed in args, or we have excess)
-						(
-							!_state.requiredInArgs(_stack[stackTop]) ||
-							_state.countInArgs(_stack[stackTop]) > _state.targetArgsCount(_stack[stackTop])
-						)
-					)
-					{
-						_stack.swap(tailOffset);
-						return {ShuffleHelperResult::Status::StackModified};
-					}
 			}
 
 			// swap up any slot in args that is out of position and has a slot available in args that it can occupy

test/libsolidity/semanticTests/externalContracts/base64.sol

@@ -41,8 +41,8 @@ contract test {
 // gas legacy code: 629800
 // gas legacyOptimized: 87926
 // gas legacyOptimized code: 429800
-// gas ssaCFGOptimized: 79931
-// gas ssaCFGOptimized code: 332600
+// gas ssaCFGOptimized: 79867
+// gas ssaCFGOptimized code: 331800
 // encode_inline_asm(bytes): 0x20, 0 -> 0x20, 0
 // encode_inline_asm(bytes): 0x20, 1, "f" -> 0x20, 4, "Zg=="
 // encode_inline_asm(bytes): 0x20, 2, "fo" -> 0x20, 4, "Zm8="
@@ -61,9 +61,9 @@ contract test {
 // gas irOptimized: 1406025
 // gas legacy: 1554038
 // gas legacyOptimized: 1132031
-// gas ssaCFGOptimized: 1388025
+// gas ssaCFGOptimized: 1386025
 // encode_no_asm_large()
 // gas irOptimized: 3512081
 // gas legacy: 4600082
 // gas legacyOptimized: 2813075
-// gas ssaCFGOptimized: 3089081
+// gas ssaCFGOptimized: 3078081

test/libsolidity/semanticTests/externalContracts/deposit_contract.sol

@@ -185,8 +185,8 @@ contract DepositContract is IDepositContract, ERC165 {
 // gas legacy code: 1438800
 // gas legacyOptimized: 848699
 // gas legacyOptimized code: 878200
-// gas ssaCFGOptimized: 809663
-// gas ssaCFGOptimized code: 570000
+// gas ssaCFGOptimized: 809711
+// gas ssaCFGOptimized code: 570600
 // supportsInterface(bytes4): 0x0 -> 0
 // supportsInterface(bytes4): 0xffffffff00000000000000000000000000000000000000000000000000000000 -> false # defined to be false by ERC-165 #
 // supportsInterface(bytes4): 0x01ffc9a700000000000000000000000000000000000000000000000000000000 -> true # ERC-165 id #

test/libsolidity/semanticTests/externalContracts/ramanujan_pi.sol

@@ -42,8 +42,8 @@ contract test {
 // gas legacy code: 523600
 // gas legacyOptimized: 82667
 // gas legacyOptimized code: 369200
-// gas ssaCFGOptimized: 77956
-// gas ssaCFGOptimized code: 313400
+// gas ssaCFGOptimized: 77788
+// gas ssaCFGOptimized code: 311000
 // prb_pi() -> 3141592656369545286
 // gas irOptimized: 55036
 // gas legacy: 100657

test/libsolidity/semanticTests/externalContracts/snark.sol

@@ -297,10 +297,10 @@ contract Test {
 // gas irOptimized: 275229
 // gas legacy: 293579
 // gas legacyOptimized: 276313
-// gas ssaCFGOptimized: 275795
+// gas ssaCFGOptimized: 275787
 // verifyTx() -> true
 // ~ emit Verified(string): 0x20, 0x16, "Successfully verified."
 // gas irOptimized: 818076
 // gas legacy: 904397
 // gas legacyOptimized: 816770
-// gas ssaCFGOptimized: 820130
+// gas ssaCFGOptimized: 820110

test/libsolidity/semanticTests/externalContracts/strings.sol

@@ -58,8 +58,8 @@ contract test {
 // gas legacy code: 932600
 // gas legacyOptimized: 102639
 // gas legacyOptimized code: 612400
-// gas ssaCFGOptimized: 96160
-// gas ssaCFGOptimized code: 532200
+// gas ssaCFGOptimized: 95989
+// gas ssaCFGOptimized code: 530000
 // toSlice(string): 0x20, 11, "hello world" -> 11, 0xa0
 // gas irOptimized: 22646
 // gas legacy: 23168
@@ -80,4 +80,4 @@ contract test {
 // gas irOptimized: 1976778
 // gas legacy: 4234020
 // gas legacyOptimized: 2318668
-// gas ssaCFGOptimized: 1882355
+// gas ssaCFGOptimized: 1841235

test/libyul/ssa/stackLayoutGenerator/nested_for.yul

@@ -107,11 +107,11 @@
 // Block0_3 [label="\
 // IN: [phi0, phi3, JUNK]\l\
 // \l\
-// [JUNK, phi3, lit2, phi0]\l\
+// [phi3, lit2, phi0]\l\
 // add\l\
-// [JUNK, phi3, v5]\l\
+// [phi3, v5]\l\
 // \l\
-// OUT: [JUNK, phi3, v5]\l\
+// OUT: [phi3, v5]\l\
 // "];
 // Block0_3 -> Block0_3Exit [arrowhead=none];
 // Block0_3Exit [label="Jump" shape=oval];

test/libyul/ssa/stackShuffler/swap_junk_over_pop.stack

@@ -8,7 +8,8 @@ targetStackSize: 4
 //    (initial)|     v0 |      *     v2
 //    PUSH lit2|     v0 |      *     v2   lit2
 //        SWAP2|     v0 |   lit2     v2      *
-//        SWAP3|      * |   lit2     v2     v0
+//          POP|     v0 |   lit2     v2
+//         DUP3|     v0 |   lit2     v2     v0
 //             +------- +---------------------
 //     (target)|     {} |   lit2     v2     v0
 // Status: Admissible