Resubmit^2: [JumpThreading] Thread jumps through two basic blocks

Resubmit^2: [JumpThreading] Thread jumps through two basic blocks

This reverts commit 41784bed01543315a1d03141e6ddc023fd914c0b.

Since the original revision ead815924e6ebeaf02c31c37ebf7a560b5fdf67b,
this revision fixes three issues:

- This revision fixes the Windows build.  My original patch improperly
  copied EH pads on Windows.  This patch disregards jump threading
  opportunities having to do with EH pads.

- This revision fixes jump threading to a wrong destination.
  Specifically, my original patch treated any Constant other than 0 as 1
  while evaluating the branch condition.  This bug led to treating
  constant expressions like:

    icmp ugt i8* null, inttoptr (i64 4 to i8*)

  to "true".  This patch fixes the bug by calling isOneValue.

- This revision fixes the cost calculation of two basic blocks being
  threaded through.  Note that getJumpThreadDuplicationCost returns
  "(unsigned)~0" for those basic blocks that cannot be duplicated.  If
  we sum of two return values from getJumpThreadDuplicationCost, we
  could have an unsigned overflow like:

    (unsigned)~0 + 5 = 4

  and mistakenly determine that it's safe and profitable to proceed
  with the jump threading opportunity.  The patch fixes the bug by
  checking each return value before summing them up.

[JumpThreading] Thread jumps through two basic blocks

Summary:
This patch teaches JumpThreading.cpp to thread through two basic
blocks like:

  bb3:
    %var = phi i32* [ null, %bb1 ], [ @a, %bb2 ]
    %tobool = icmp eq i32 %cond, 0
    br i1 %tobool, label %bb4, label ...

  bb4:
    %cmp = icmp eq i32* %var, null
    br i1 %cmp, label bb5, label bb6

by duplicating basic blocks like bb3 above.  Once we duplicate bb3 as
bb3.dup and redirect edge bb2->bb3 to bb2->bb3.dup, we have:

  bb3:
    %var = phi i32* [ @a, %bb2 ]
    %tobool = icmp eq i32 %cond, 0
    br i1 %tobool, label %bb4, label ...

  bb3.dup:
    %var = phi i32* [ null, %bb1 ]
    %tobool = icmp eq i32 %cond, 0
    br i1 %tobool, label %bb4, label ...

  bb4:
    %cmp = icmp eq i32* %var, null
    br i1 %cmp, label bb5, label bb6

Then the existing code in JumpThreading.cpp can thread edge
bb3.dup->bb4 through bb4 and eventually create bb3.dup->bb5.

Reviewers: wmi

Subscribers: hiraditya, jfb, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D70247