[JumpThreading] Unfold selects that depend on the same condition

Summary:
These are good candidates for jump threading. This enables later opts
(such as InstCombine) to combine instructions from the selects with
instructions out of the selects. SimplifyCFG will fold the select
again if unfolding wasn't worth it.

Patch by James Molloy and Pablo Barrio.

Reviewers: rengolin, haicheng, sebpop

Subscribers: jojo, jmolloy, llvm-commits

Differential Revision: https://reviews.llvm.org/D26391

llvm-svn: 286236

1 files changed, 45 insertions, 0 deletions

diff --git a/llvm/test/Transforms/JumpThreading/unfold-selects-same-cond.ll b/llvm/test/Transforms/JumpThreading/unfold-selects-same-cond.ll
new file mode 100644
index 00000000000..ef01b0d1990
--- /dev/null
+++ b/llvm/test/Transforms/JumpThreading/unfold-selects-same-cond.ll
@@ -0,0 +1,45 @@
+; RUN: opt < %s -jump-threading -instcombine -simplifycfg  -S | FileCheck %s
+
+; The three selects are jump-threaded so that instcombine can optimize, and
+; simplifycfg should turn the result into a single select.
+define i32 @f(i32 %a, i32 %b) {
+; CHECK: select
+; CHECK-NOT: select
+entry:
+  %0 = and i32 %a, 1
+  %1 = and i32 %b, 1
+  %xor = xor i32 %1, %a
+  %shr32 = lshr i32 %a, 1
+  %cmp10 = icmp eq i32 %xor, 1
+  %2 = xor i32 %b, 12345
+  %b.addr.1 = select i1 %cmp10, i32 %2, i32 %b
+  %shr1633 = lshr i32 %b.addr.1, 1
+  %3 = or i32 %shr1633, 54321
+  %b.addr.2 = select i1 %cmp10, i32 %3, i32 %shr1633
+  %shr1634 = lshr i32 %b.addr.2, 2
+  %4 = or i32 %shr1634, 54320
+  %b.addr.3 = select i1 %cmp10, i32 %4, i32 %shr1634
+  ret i32 %b.addr.3
+}
+
+; Case where the condition is not only used as condition but also as the
+; true or false value in at least one of the selects.
+define i1 @g(i32 %a, i32 %b) {
+; CHECK: select
+; CHECK-NOT: select
+entry:
+  %0 = and i32 %a, 1
+  %1 = and i32 %b, 1
+  %xor = xor i32 %1, %a
+  %shr32 = lshr i32 %a, 1
+  %cmp10 = icmp eq i32 %xor, 1
+  %2 = xor i32 %b, 12345
+  %b.addr.1 = select i1 %cmp10, i32 %2, i32 %b
+  %shr1633 = lshr i32 %b.addr.1, 1
+  %3 = or i32 %shr1633, 54321
+  %b.addr.2 = select i1 %cmp10, i32 %3, i32 %shr1633
+  %shr1634 = lshr i32 %b.addr.2, 2
+  %4 = icmp eq i32 %shr1634, 54320
+  %b.addr.3 = select i1 %cmp10, i1 %4, i1 %cmp10
+  ret i1 %b.addr.3
+}