ASTNode::simplify has supralinear performance with deep nesting of expressions

[whitequark]

Version

Yosys 0.39+1 (git sha1 f7153573c, ccache clang++ 14.0.6 -O0 -fPIC)

On which OS did this happen?

Linux

Reproduction Steps

Run yosys stackoverflow.v -p hierarchy while commenting out some of the 512 levels of nesting (with stackoverflow.v being the example from YoWASP/yosys#37) and observe time plus stack usasge, for example with valgrind --tool=drd --main-stacksize=16777216 --show-stack-usage=yes.

Expected Behavior

Time is linear in input size.

Actual Behavior

Time is quadratic in input size:

_{(link to the spreadsheet)}

LibreOffice Calc provides the following polynomial fit:

Also, using 24.5K per nesting level seems excessive and results in issues such as YoWASP/yosys#37.

[widlarizer]

using 24.5K per nesting level

I ran massif on stackoverflow.v. In the peak snapshot, I can see

92.53% (3,345,110B) (heap allocation functions) malloc/new/new[], --alloc-fns, etc.
->32.73% (1,183,104B) 0x720AB4: Yosys::AST::AstNode::clone() const (frontends/ast/ast.cc:253)
| ->32.69% (1,181,664B) 0x720BB7: Yosys::AST::AstNode::clone() const (frontends/ast/ast.cc:256)
| | ->32.63% (1,179,648B) 0x720BB7: Yosys::AST::AstNode::clone() const (frontends/ast/ast.cc:256)

and so on, there's a lot of deep copies of nodes. There's also a lot of memory spent on parser-constructed AstNodes. Edited massif.log. The sizeof(AstNode) is 288B according to a dump I did some weeks back. This means we need 85 AstNodes per ternary nesting level.

[whitequark]

That's 24.5K of stack memory, not heap memory. (I'm sure heap use isn't great either, but that's not what I'm talking about.)

You only get 1M of stack on Windows and macOS, so beyond 80 levels or so Yosys crashes.

[widlarizer]

...right, of course. valgrind --tool=massif --heap=no --stacks=yes ./yosys garbage/stackoverflow.v -p "hierarchy" says we get up to 1,335,808 bytes of stack, but can't attribute it to its sources. perf shows that AstNode::detectSignWidthWorker takes up 90% of the runtime.

[widlarizer]

Just noticed that we're raising the stack size in a linux ifdef in kernel/rtlil.cc