[llvm] [InstCombine] Fold `umax(X, C) + -C` into `usub.sat(X, C)` (PR #118195)

[Yingwei Zheng via llvm-commits]

https://github.com/dtcxzyw created https://github.com/llvm/llvm-project/pull/118195

Alive2: https://alive2.llvm.org/ce/z/oSWe5S
Closes https://github.com/llvm/llvm-project/issues/118155


>From 6e8cebe50c136113d230e6a243425e677a761536 Mon Sep 17 00:00:00 2001
From: Yingwei Zheng <dtcxzyw2333 at gmail.com>
Date: Sun, 1 Dec 2024 14:11:24 +0800
Subject: [PATCH 1/2] [InstCombine] Add pre-commit tests. NFC.

---
 .../InstCombine/saturating-add-sub.ll         | 37 +++++++++++++++++++
 1 file changed, 37 insertions(+)

diff --git a/llvm/test/Transforms/InstCombine/saturating-add-sub.ll b/llvm/test/Transforms/InstCombine/saturating-add-sub.ll
index 9236d96f59a55b..e75f4674f22eb0 100644
--- a/llvm/test/Transforms/InstCombine/saturating-add-sub.ll
+++ b/llvm/test/Transforms/InstCombine/saturating-add-sub.ll
@@ -2316,3 +2316,40 @@ define i32 @uadd_sat_via_add_swapped_cmp_select_nonstrict(i32 %x, i32 %y) {
   %r = select i1 %c, i32 %a, i32 -1
   ret i32 %r
 }
+
+define i8 @fold_add_umax_to_usub(i8 %a) {
+; CHECK-LABEL: @fold_add_umax_to_usub(
+; CHECK-NEXT:    [[UMAX:%.*]] = call i8 @llvm.umax.i8(i8 [[A:%.*]], i8 10)
+; CHECK-NEXT:    [[SEL:%.*]] = add i8 [[UMAX]], -10
+; CHECK-NEXT:    ret i8 [[SEL]]
+;
+  %umax = call i8 @llvm.umax.i8(i8 %a, i8 10)
+  %sel = add i8 %umax, -10
+  ret i8 %sel
+}
+
+define i8 @fold_add_umax_to_usub_incorrect_rhs(i8 %a) {
+; CHECK-LABEL: @fold_add_umax_to_usub_incorrect_rhs(
+; CHECK-NEXT:    [[UMAX:%.*]] = call i8 @llvm.umax.i8(i8 [[A:%.*]], i8 10)
+; CHECK-NEXT:    [[SEL:%.*]] = add i8 [[UMAX]], -11
+; CHECK-NEXT:    ret i8 [[SEL]]
+;
+  %umax = call i8 @llvm.umax.i8(i8 %a, i8 10)
+  %sel = add i8 %umax, -11
+  ret i8 %sel
+}
+
+define i8 @fold_add_umax_to_usub_multiuse(i8 %a) {
+; CHECK-LABEL: @fold_add_umax_to_usub_multiuse(
+; CHECK-NEXT:    [[UMAX:%.*]] = call i8 @llvm.umax.i8(i8 [[A:%.*]], i8 10)
+; CHECK-NEXT:    call void @usei8(i8 [[UMAX]])
+; CHECK-NEXT:    [[SEL:%.*]] = add i8 [[UMAX]], -10
+; CHECK-NEXT:    ret i8 [[SEL]]
+;
+  %umax = call i8 @llvm.umax.i8(i8 %a, i8 10)
+  call void @usei8(i8 %umax)
+  %sel = add i8 %umax, -10
+  ret i8 %sel
+}
+
+declare void @usei8(i8)

>From 60d830e24a123f8d380cb29ac156d8336e3ce41a Mon Sep 17 00:00:00 2001
From: Yingwei Zheng <dtcxzyw2333 at gmail.com>
Date: Sun, 1 Dec 2024 14:22:00 +0800
Subject: [PATCH 2/2] [InstCombine] Fold `umax(X, C) + -C` into `usub.sat(X,
 C)`

---
 llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp  | 6 ++++++
 llvm/test/Transforms/InstCombine/saturating-add-sub.ll | 3 +--
 2 files changed, 7 insertions(+), 2 deletions(-)

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp b/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
index 6fe96935818531..63725e4ca8113e 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
@@ -994,6 +994,12 @@ Instruction *InstCombinerImpl::foldAddWithConstant(BinaryOperator &Add) {
     }
   }
 
+  // umax(X, C) + -C --> usub.sat(X, C)
+  if (match(Op0, m_OneUse(m_UMax(m_Value(X), m_SpecificInt(-*C)))))
+    return replaceInstUsesWith(
+        Add, Builder.CreateBinaryIntrinsic(
+                 Intrinsic::usub_sat, X, ConstantInt::get(Add.getType(), -*C)));
+
   // Fold (add (zext (add X, -1)), 1) -> (zext X) if X is non-zero.
   // TODO: There's a general form for any constant on the outer add.
   if (C->isOne()) {
diff --git a/llvm/test/Transforms/InstCombine/saturating-add-sub.ll b/llvm/test/Transforms/InstCombine/saturating-add-sub.ll
index e75f4674f22eb0..d043f5b7ad116d 100644
--- a/llvm/test/Transforms/InstCombine/saturating-add-sub.ll
+++ b/llvm/test/Transforms/InstCombine/saturating-add-sub.ll
@@ -2319,8 +2319,7 @@ define i32 @uadd_sat_via_add_swapped_cmp_select_nonstrict(i32 %x, i32 %y) {
 
 define i8 @fold_add_umax_to_usub(i8 %a) {
 ; CHECK-LABEL: @fold_add_umax_to_usub(
-; CHECK-NEXT:    [[UMAX:%.*]] = call i8 @llvm.umax.i8(i8 [[A:%.*]], i8 10)
-; CHECK-NEXT:    [[SEL:%.*]] = add i8 [[UMAX]], -10
+; CHECK-NEXT:    [[SEL:%.*]] = call i8 @llvm.usub.sat.i8(i8 [[A:%.*]], i8 10)
 ; CHECK-NEXT:    ret i8 [[SEL]]
 ;
   %umax = call i8 @llvm.umax.i8(i8 %a, i8 10)