[PATCH] [CodeGenPrepare] Teach when it is profitable to speculate calls to @llvm.cttz/ctlz.

[Andrea Di Biagio]

Hi qcolombet, hfinkel, majnemer,

Following up on the suggestions from D6679, here is a new patch to optimize cttz/ctlz in CodeGenPrepare.

If we know that the control flow is modelling an if-statement where the only instruction in 'then' basic block (excluding the terminator) is a call to cttz/ctlz, CodeGenPrepare can try to speculate the cttz/ctlz call and simplify the control flow graph.

Example:
;;
entry:
  %cmp = icmp eq i64 %Val, 0
  br i1 %cmp, label %end.bb, label %then.bb

then.bb:
  %c = tail call i64 @llvm.cttz.i64(i64 %val, i1 true)
  br label %EndBB

end.bb:
  %cond = phi i64 [ %c, %then.bb ], [ 64, %entry]
  ...
;;

The call to @llvm.cttz.i64 could be speculated. This would allow to get rid of 'then.bb' and merge %entry.bb with %end.bb.

The constraints are:
a) The 'then' basic block is taken only if the input operand to the cttz/ctlz is different than zero;
b) The phi node propagates the size-of (in bits) of the value %val in input to the cttz/ctlz if %val is zero.
c) The target says that it is "cheap" to speculate cttz/ctlz.

If all these constraints are met, CodeGenPrepare can hoist the call to cttz/ctlz from the 'then' basic block into the 'entry' basic block. The new cttz/ctlz instruction will also have the 'undef on zero' flag set to 'false'.

I added two new hooks in TargetLowering.h to let targets customize the behavior (i.e. decide whether it is cheap or not to speculate calls to cttz/ctlz). The two new methods are 'isCheapToSpeculateCtlz' and 'isCheapToSpeculateCttz'.
By default, both methods return 'false'. Which means, CodeGenPrepare doesn't try to speculate calls to cttz/ctlz unless the target says that it is profitable to do it.

On X86, method 'isCheapToSpeculateCtlz' returns true only if the target has LZCNT. Method 'isCheapToSpeculateCttz' only returns true if the target has BMI.
This may change in future. For now, I avoided to enable the transformation for all x86-64 targets with feature CMOV because I am not 100% it is always a win to speculate bsf/bsr. So, I left a couple of TODO comments in the code.

Please let me know what you think.

Thanks!
Andrea

http://reviews.llvm.org/D6728

Files:
  include/llvm/Target/TargetLowering.h
  lib/CodeGen/CodeGenPrepare.cpp
  lib/Target/X86/X86ISelLowering.cpp
  lib/Target/X86/X86ISelLowering.h
  test/CodeGen/X86/cttz-ctlz.ll

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