#8275: Loopification breaks profiling ----------------------------------------+---------------------------------- Reporter: jstolarek | Owner: jstolarek Type: bug | Status: new Priority: high | Milestone: 7.8.1 Component: Profiling | Version: 7.7 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: Building GHC failed | Unknown/Multiple Test Case: | Difficulty: Unknown Blocking: 8298 | Blocked By: 8456 | Related Tickets: ----------------------------------------+---------------------------------- Comment (by jstolarek): Replying to [comment:24 simonpj]:
But I don't understand the patch (in comment 19), to `CmmLayoutStack`. The change made there is simply to ''pass a different (larger) value to `manifestSp`''. And that's the only change to that module. So we'll generate different, wrong code. I don't see how this can possibly work. And yet apparently it does. What am I missing?
I'll try to explain this with an example of function that uses no stack space. Previously we generated stack checks that looked like this: {{{ if (Sp - <highSp> < SpLim) ... }}} If the function used no stack, then stack layout (`areaToSp` function, second guard) transformed this into: {{{ if (Sp - 0 < SpLim) ... }}} This check is always false and it used to be eliminated by the third guard in `areaToSp`. The value of `<highSp>` is computed to be `0` by the line of code that I removed in !CmmLayoutStack. Now consider new version. Code generator gives us: {{{ if ((old + 0) - <highSp> < SpLim) ... }}} As you pointed out, `<highSp>` will now be larger. But `(old + 0)` will not be `Sp`! This check will be turned into something this: {{{ if ((Sp + 8) - 8 < SpLim) ... }}} That is the reason why the elimination of always false stack checks was killed by my change. BTW. Simon, you wrote the code for that patch when I was in Cambridge :-) Do you remember how we (well, mostly you) updated [http://ghc.haskell.org/trac/ghc/wiki/Commentary/Compiler/StackAreas this wiki page] and how we were confused with addressing offsets into an `Old` area? -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8275#comment:26 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler