[llvm] [DAGCombiner] Fold subtraction if above threshold to `umin` (PR #134235)

[Piotr Fusik via llvm-commits]

================
@@ -1699,3 +1699,209 @@ entry:
   %cmp = icmp eq i32 %popcnt, 1
   ret i1 %cmp
 }
+
+define i8 @sub_if_uge_i8(i8 %x, i8 %y) {
+; RV64I-LABEL: sub_if_uge_i8:
+; RV64I:       # %bb.0:
+; RV64I-NEXT:    zext.b a2, a1
+; RV64I-NEXT:    zext.b a3, a0
+; RV64I-NEXT:    sltu a2, a3, a2
+; RV64I-NEXT:    addi a2, a2, -1
+; RV64I-NEXT:    and a1, a2, a1
+; RV64I-NEXT:    sub a0, a0, a1
+; RV64I-NEXT:    ret
+;
+; RV64ZBB-LABEL: sub_if_uge_i8:
+; RV64ZBB:       # %bb.0:
+; RV64ZBB-NEXT:    zext.b a2, a0
+; RV64ZBB-NEXT:    subw a0, a0, a1
+; RV64ZBB-NEXT:    zext.b a0, a0
+; RV64ZBB-NEXT:    minu a0, a2, a0
+; RV64ZBB-NEXT:    ret
+  %cmp = icmp ult i8 %x, %y
+  %select = select i1 %cmp, i8 0, i8 %y
+  %sub = sub nuw i8 %x, %select
+  ret i8 %sub
+}
+
+define i16 @sub_if_uge_i16(i16 %x, i16 %y) {
+; RV64I-LABEL: sub_if_uge_i16:
+; RV64I:       # %bb.0:
+; RV64I-NEXT:    lui a2, 16
+; RV64I-NEXT:    addiw a2, a2, -1
+; RV64I-NEXT:    and a3, a1, a2
+; RV64I-NEXT:    and a2, a0, a2
+; RV64I-NEXT:    sltu a2, a2, a3
+; RV64I-NEXT:    addi a2, a2, -1
+; RV64I-NEXT:    and a1, a2, a1
+; RV64I-NEXT:    sub a0, a0, a1
+; RV64I-NEXT:    ret
+;
+; RV64ZBB-LABEL: sub_if_uge_i16:
+; RV64ZBB:       # %bb.0:
+; RV64ZBB-NEXT:    zext.h a2, a0
+; RV64ZBB-NEXT:    subw a0, a0, a1
+; RV64ZBB-NEXT:    zext.h a0, a0
+; RV64ZBB-NEXT:    minu a0, a2, a0
+; RV64ZBB-NEXT:    ret
+  %cmp = icmp ult i16 %x, %y
+  %select = select i1 %cmp, i16 0, i16 %y
+  %sub = sub nuw i16 %x, %select
+  ret i16 %sub
+}
+
+define i32 @sub_if_uge_i32(i32 %x, i32 %y) {
+; RV64I-LABEL: sub_if_uge_i32:
+; RV64I:       # %bb.0:
+; RV64I-NEXT:    sext.w a2, a1
+; RV64I-NEXT:    sext.w a3, a0
+; RV64I-NEXT:    sltu a2, a3, a2
+; RV64I-NEXT:    addi a2, a2, -1
+; RV64I-NEXT:    and a1, a2, a1
+; RV64I-NEXT:    subw a0, a0, a1
+; RV64I-NEXT:    ret
+;
+; RV64ZBB-LABEL: sub_if_uge_i32:
+; RV64ZBB:       # %bb.0:
+; RV64ZBB-NEXT:    sext.w a2, a0
+; RV64ZBB-NEXT:    subw a0, a0, a1
+; RV64ZBB-NEXT:    minu a0, a2, a0
+; RV64ZBB-NEXT:    ret
+  %cmp = icmp ult i32 %x, %y
+  %select = select i1 %cmp, i32 0, i32 %y
+  %sub = sub nuw i32 %x, %select
+  ret i32 %sub
+}
+
+define i64 @sub_if_uge_i64(i64 %x, i64 %y) {
+; RV64I-LABEL: sub_if_uge_i64:
+; RV64I:       # %bb.0:
+; RV64I-NEXT:    sltu a2, a0, a1
+; RV64I-NEXT:    addi a2, a2, -1
+; RV64I-NEXT:    and a1, a2, a1
+; RV64I-NEXT:    sub a0, a0, a1
+; RV64I-NEXT:    ret
+;
+; RV64ZBB-LABEL: sub_if_uge_i64:
+; RV64ZBB:       # %bb.0:
+; RV64ZBB-NEXT:    sub a1, a0, a1
+; RV64ZBB-NEXT:    minu a0, a0, a1
+; RV64ZBB-NEXT:    ret
+  %cmp = icmp ult i64 %x, %y
+  %select = select i1 %cmp, i64 0, i64 %y
+  %sub = sub nuw i64 %x, %select
+  ret i64 %sub
+}
+
+define i128 @sub_if_uge_i128(i128 %x, i128 %y) {
+; RV64I-LABEL: sub_if_uge_i128:
+; RV64I:       # %bb.0:
+; RV64I-NEXT:    beq a1, a3, .LBB66_2
+; RV64I-NEXT:  # %bb.1:
+; RV64I-NEXT:    sltu a4, a1, a3
+; RV64I-NEXT:    j .LBB66_3
+; RV64I-NEXT:  .LBB66_2:
+; RV64I-NEXT:    sltu a4, a0, a2
+; RV64I-NEXT:  .LBB66_3:
+; RV64I-NEXT:    addi a4, a4, -1
+; RV64I-NEXT:    and a3, a4, a3
+; RV64I-NEXT:    and a2, a4, a2
+; RV64I-NEXT:    sltu a4, a0, a2
+; RV64I-NEXT:    sub a1, a1, a3
+; RV64I-NEXT:    sub a1, a1, a4
+; RV64I-NEXT:    sub a0, a0, a2
+; RV64I-NEXT:    ret
+;
+; RV64ZBB-LABEL: sub_if_uge_i128:
+; RV64ZBB:       # %bb.0:
+; RV64ZBB-NEXT:    beq a1, a3, .LBB66_2
+; RV64ZBB-NEXT:  # %bb.1:
+; RV64ZBB-NEXT:    sltu a4, a1, a3
+; RV64ZBB-NEXT:    j .LBB66_3
+; RV64ZBB-NEXT:  .LBB66_2:
+; RV64ZBB-NEXT:    sltu a4, a0, a2
+; RV64ZBB-NEXT:  .LBB66_3:
+; RV64ZBB-NEXT:    addi a4, a4, -1
+; RV64ZBB-NEXT:    and a3, a4, a3
+; RV64ZBB-NEXT:    and a2, a4, a2
+; RV64ZBB-NEXT:    sltu a4, a0, a2
+; RV64ZBB-NEXT:    sub a1, a1, a3
+; RV64ZBB-NEXT:    sub a1, a1, a4
+; RV64ZBB-NEXT:    sub a0, a0, a2
+; RV64ZBB-NEXT:    ret
+  %cmp = icmp ult i128 %x, %y
+  %select = select i1 %cmp, i128 0, i128 %y
+  %sub = sub nuw i128 %x, %select
+  ret i128 %sub
+}
+
+define i32 @sub_if_uge_multiuse_select_i32(i32 %x, i32 %y) {
+; RV64I-LABEL: sub_if_uge_multiuse_select_i32:
+; RV64I:       # %bb.0:
+; RV64I-NEXT:    sext.w a2, a1
+; RV64I-NEXT:    sext.w a3, a0
+; RV64I-NEXT:    sltu a2, a3, a2
+; RV64I-NEXT:    addi a2, a2, -1
+; RV64I-NEXT:    and a1, a2, a1
+; RV64I-NEXT:    subw a0, a0, a1
+; RV64I-NEXT:    sllw a0, a0, a1
+; RV64I-NEXT:    ret
+;
+; RV64ZBB-LABEL: sub_if_uge_multiuse_select_i32:
+; RV64ZBB:       # %bb.0:
+; RV64ZBB-NEXT:    sext.w a2, a1
+; RV64ZBB-NEXT:    sext.w a3, a0
+; RV64ZBB-NEXT:    sltu a2, a3, a2
+; RV64ZBB-NEXT:    addi a2, a2, -1
+; RV64ZBB-NEXT:    and a1, a2, a1
+; RV64ZBB-NEXT:    subw a0, a0, a1
+; RV64ZBB-NEXT:    sllw a0, a0, a1
+; RV64ZBB-NEXT:    ret
+  %cmp = icmp ult i32 %x, %y
+  %select = select i1 %cmp, i32 0, i32 %y
+  %sub = sub nuw i32 %x, %select
+  %shl = shl i32 %sub, %select
+  ret i32 %shl
+}
+
+define i32 @sub_if_uge_multiuse_cmp_i32(i32 %x, i32 %y) {
+; RV64I-LABEL: sub_if_uge_multiuse_cmp_i32:
+; RV64I:       # %bb.0:
+; RV64I-NEXT:    sext.w a2, a1
+; RV64I-NEXT:    sext.w a3, a0
+; RV64I-NEXT:    sltu a4, a3, a2
+; RV64I-NEXT:    addi a4, a4, -1
+; RV64I-NEXT:    and a1, a4, a1
+; RV64I-NEXT:    subw a0, a0, a1
+; RV64I-NEXT:    bltu a3, a2, .LBB68_2
+; RV64I-NEXT:  # %bb.1:
+; RV64I-NEXT:    li a1, 4
+; RV64I-NEXT:    sllw a0, a0, a1
+; RV64I-NEXT:    ret
+; RV64I-NEXT:  .LBB68_2:
+; RV64I-NEXT:    li a1, 2
+; RV64I-NEXT:    sllw a0, a0, a1
+; RV64I-NEXT:    ret
+;
+; RV64ZBB-LABEL: sub_if_uge_multiuse_cmp_i32:
+; RV64ZBB:       # %bb.0:
+; RV64ZBB-NEXT:    sext.w a2, a1
----------------
pfusik wrote:

Good idea, added.

https://github.com/llvm/llvm-project/pull/134235