[InstCombiner] Expose opportunities to merge subtract and comparison.

Several architectures use the same instruction to perform both a comparison and
a subtract. The instruction selection framework does not allow to consider
different basic blocks to expose such fusion opportunities.

Therefore, these instructions are “merged” by CSE at MI IR level.

To increase the likelihood of CSE to apply in such situation, we reorder the
operands of the comparison, when they have the same complexity, so that they
matches the order of the most frequent subtract.
E.g.,

icmp A, B
...
sub B, A

<rdar://problem/14514580>

llvm-svn: 190352

1 files changed, 65 insertions, 0 deletions

diff --git a/llvm/test/Transforms/InstCombine/icmp.ll b/llvm/test/Transforms/InstCombine/icmp.ll
index 33636c47d3d..bd8128ffb1b 100644
--- a/llvm/test/Transforms/InstCombine/icmp.ll
+++ b/llvm/test/Transforms/InstCombine/icmp.ll
@@ -1271,3 +1271,68 @@ define i1 @icmp_sub_-1_X_uge_4(i32 %X) {
   %cmp = icmp uge i32 %sub, 4
   ret i1 %cmp
 }
+
+; CHECK-LABEL: @icmp_swap_operands_for_cse
+; CHECK: [[CMP:%[a-z0-9]+]] = icmp ult i32 %X, %Y
+; CHECK-NEXT: br i1 [[CMP]], label %true, label %false
+; CHECK: ret i1
+define i1 @icmp_swap_operands_for_cse(i32 %X, i32 %Y) {
+entry:
+  %sub = sub i32 %X, %Y
+  %cmp = icmp ugt i32 %Y, %X
+  br i1 %cmp, label %true, label %false
+true:
+  %restrue = trunc i32 %sub to i1
+  br label %end
+false:
+  %shift = lshr i32 %sub, 4
+  %resfalse = trunc i32 %shift to i1
+  br label %end
+end:
+  %res = phi i1 [%restrue, %true], [%resfalse, %false]
+  ret i1 %res
+}
+
+; CHECK-LABEL: @icmp_swap_operands_for_cse2
+; CHECK: [[CMP:%[a-z0-9]+]] = icmp ult i32 %X, %Y
+; CHECK-NEXT: br i1 [[CMP]], label %true, label %false
+; CHECK: ret i1
+define i1 @icmp_swap_operands_for_cse2(i32 %X, i32 %Y) {
+entry:
+  %cmp = icmp ugt i32 %Y, %X
+  br i1 %cmp, label %true, label %false
+true:
+  %sub = sub i32 %X, %Y
+  %sub1 = sub i32 %X, %Y
+  %add = add i32 %sub, %sub1
+  %restrue = trunc i32 %add to i1
+  br label %end
+false:
+  %sub2 = sub i32 %Y, %X
+  %resfalse = trunc i32 %sub2 to i1
+  br label %end
+end:
+  %res = phi i1 [%restrue, %true], [%resfalse, %false]
+  ret i1 %res
+}
+
+; CHECK-LABEL: @icmp_do_not_swap_operands_for_cse
+; CHECK: [[CMP:%[a-z0-9]+]] = icmp ugt i32 %Y, %X
+; CHECK-NEXT: br i1 [[CMP]], label %true, label %false
+; CHECK: ret i1
+define i1 @icmp_do_not_swap_operands_for_cse(i32 %X, i32 %Y) {
+entry:
+  %cmp = icmp ugt i32 %Y, %X
+  br i1 %cmp, label %true, label %false
+true:
+  %sub = sub i32 %X, %Y
+  %restrue = trunc i32 %sub to i1
+  br label %end
+false:
+  %sub2 = sub i32 %Y, %X
+  %resfalse = trunc i32 %sub2 to i1
+  br label %end
+end:
+  %res = phi i1 [%restrue, %true], [%resfalse, %false]
+  ret i1 %res
+}