InstCombine: (icmp eq B, 0) | (icmp ult A, B) -> (icmp ule A, B-1)

This transform allows us to turn IR that looks like:
  %1 = icmp eq i64 %b, 0
  %2 = icmp ult i64 %a, %b
  %3 = or i1 %1, %2
  ret i1 %3

into:
  %0 = add i64 %b, -1
  %1 = icmp uge i64 %0, %a
  ret i1 %1

which means we go from lowering:
        cmpq    %rsi, %rdi
        setb    %cl
        testq   %rsi, %rsi
        sete    %al
        orb     %cl, %al
        ret

to lowering:
        decq    %rsi
        cmpq    %rdi, %rsi
        setae   %al
        ret

llvm-svn: 185677

1 files changed, 11 insertions, 0 deletions

diff --git a/llvm/test/Transforms/InstCombine/icmp.ll b/llvm/test/Transforms/InstCombine/icmp.ll
index 0026b20696c..195dc572adf 100644
--- a/llvm/test/Transforms/InstCombine/icmp.ll
+++ b/llvm/test/Transforms/InstCombine/icmp.ll
@@ -1102,3 +1102,14 @@ define i1 @icmp_shl_1_V_ult_2147483648(i32 %V) {
   %cmp = icmp ult i32 %shl, 2147483648
   ret i1 %cmp
 }
+
+; CEHCK: @or_icmp_eq_B_0_icmp_ult_A_B
+; CHECK: [[SUB:%[a-z0-9]+]] = add i64 %b, -1
+; CHECK-NEXT: [[CMP:%[a-z0-9]+]] = icmp uge i64 [[SUB]], %a
+; CHECK-NEXT: ret i1 [[CMP]]
+define i1 @or_icmp_eq_B_0_icmp_ult_A_B(i64 %a, i64 %b) {
+  %1 = icmp eq i64 %b, 0
+  %2 = icmp ult i64 %a, %b
+  %3 = or i1 %1, %2
+  ret i1 %3
+}