[llvm] [InstCombine] Fold `scmp(x -nsw y, 0` to `scmp(x, y)` (PR #105583)

[Volodymyr Vasylkun via llvm-commits]

https://github.com/Poseydon42 created https://github.com/llvm/llvm-project/pull/105583

Proof: https://alive2.llvm.org/ce/z/v6VtXz

>From cf6cbb7bb9e581c558d0e6c32e657995a5263f82 Mon Sep 17 00:00:00 2001
From: Poseydon42 <vvmposeydon at gmail.com>
Date: Wed, 21 Aug 2024 21:38:55 +0100
Subject: [PATCH 1/2] Precommit tests

---
 llvm/test/Transforms/InstCombine/scmp.ll | 52 ++++++++++++++++++++++++
 1 file changed, 52 insertions(+)

diff --git a/llvm/test/Transforms/InstCombine/scmp.ll b/llvm/test/Transforms/InstCombine/scmp.ll
index 7f374c5f9a1d64..79b38652185e74 100644
--- a/llvm/test/Transforms/InstCombine/scmp.ll
+++ b/llvm/test/Transforms/InstCombine/scmp.ll
@@ -264,3 +264,55 @@ define i8 @scmp_from_select_ge(i32 %x, i32 %y) {
   %r = select i1 %ge, i8 %ne, i8 -1
   ret i8 %r
 }
+
+; Fold scmp(x nsw- y, 0) to scmp(x, y)
+define i8 @scmp_of_sub_and_zero(i32 %x, i32 %y) {
+; CHECK-LABEL: define i8 @scmp_of_sub_and_zero(
+; CHECK-SAME: i32 [[X:%.*]], i32 [[Y:%.*]]) {
+; CHECK-NEXT:    [[DIFF:%.*]] = sub nsw i32 [[X]], [[Y]]
+; CHECK-NEXT:    [[R:%.*]] = call i8 @llvm.scmp.i8.i32(i32 [[DIFF]], i32 0)
+; CHECK-NEXT:    ret i8 [[R]]
+;
+  %diff = sub nsw i32 %x, %y
+  %r = call i8 @llvm.scmp(i32 %diff, i32 0)
+  ret i8 %r
+}
+
+; Negative test: no nsw
+define i8 @scmp_of_sub_and_zero_neg_1(i32 %x, i32 %y) {
+; CHECK-LABEL: define i8 @scmp_of_sub_and_zero_neg_1(
+; CHECK-SAME: i32 [[X:%.*]], i32 [[Y:%.*]]) {
+; CHECK-NEXT:    [[DIFF:%.*]] = sub i32 [[X]], [[Y]]
+; CHECK-NEXT:    [[R:%.*]] = call i8 @llvm.scmp.i8.i32(i32 [[DIFF]], i32 0)
+; CHECK-NEXT:    ret i8 [[R]]
+;
+  %diff = sub i32 %x, %y
+  %r = call i8 @llvm.scmp(i32 %diff, i32 0)
+  ret i8 %r
+}
+
+; Negative test: second argument of scmp is not 0
+define i8 @scmp_of_sub_and_zero_neg2(i32 %x, i32 %y) {
+; CHECK-LABEL: define i8 @scmp_of_sub_and_zero_neg2(
+; CHECK-SAME: i32 [[X:%.*]], i32 [[Y:%.*]]) {
+; CHECK-NEXT:    [[DIFF:%.*]] = sub nsw i32 [[X]], [[Y]]
+; CHECK-NEXT:    [[R:%.*]] = call i8 @llvm.scmp.i8.i32(i32 [[DIFF]], i32 15)
+; CHECK-NEXT:    ret i8 [[R]]
+;
+  %diff = sub nsw i32 %x, %y
+  %r = call i8 @llvm.scmp(i32 %diff, i32 15)
+  ret i8 %r
+}
+
+; Negative test: calling ucmp instead of scmp
+define i8 @scmp_of_sub_and_zero_neg3(i32 %x, i32 %y) {
+; CHECK-LABEL: define i8 @scmp_of_sub_and_zero_neg3(
+; CHECK-SAME: i32 [[X:%.*]], i32 [[Y:%.*]]) {
+; CHECK-NEXT:    [[DIFF:%.*]] = sub nsw i32 [[X]], [[Y]]
+; CHECK-NEXT:    [[R:%.*]] = call i8 @llvm.ucmp.i8.i32(i32 [[DIFF]], i32 0)
+; CHECK-NEXT:    ret i8 [[R]]
+;
+  %diff = sub nsw i32 %x, %y
+  %r = call i8 @llvm.ucmp(i32 %diff, i32 0)
+  ret i8 %r
+}

>From ce0be8237ae67cde5741264cb423ce3ba815e2e6 Mon Sep 17 00:00:00 2001
From: Poseydon42 <vvmposeydon at gmail.com>
Date: Wed, 21 Aug 2024 21:39:35 +0100
Subject: [PATCH 2/2] Implement the fold

---
 llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp | 9 +++++++++
 llvm/test/Transforms/InstCombine/scmp.ll             | 3 +--
 2 files changed, 10 insertions(+), 2 deletions(-)

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
index 2c2e1bc4686a4e..eb94e894b57b06 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
@@ -1929,6 +1929,15 @@ Instruction *InstCombinerImpl::visitCallInst(CallInst &CI) {
 
     break;
   }
+  case Intrinsic::scmp: {
+    Value *I0 = II->getArgOperand(0), *I1 = II->getArgOperand(1);
+    Value *LHS, *RHS;
+    if (match(I0, m_NSWSub(m_Value(LHS), m_Value(RHS))) && match(I1, m_Zero()))
+      return replaceInstUsesWith(
+          CI,
+          Builder.CreateIntrinsic(II->getType(), Intrinsic::scmp, {LHS, RHS}));
+    break;
+  }
   case Intrinsic::bitreverse: {
     Value *IIOperand = II->getArgOperand(0);
     // bitrev (zext i1 X to ?) --> X ? SignBitC : 0
diff --git a/llvm/test/Transforms/InstCombine/scmp.ll b/llvm/test/Transforms/InstCombine/scmp.ll
index 79b38652185e74..a3334599a67f1c 100644
--- a/llvm/test/Transforms/InstCombine/scmp.ll
+++ b/llvm/test/Transforms/InstCombine/scmp.ll
@@ -269,8 +269,7 @@ define i8 @scmp_from_select_ge(i32 %x, i32 %y) {
 define i8 @scmp_of_sub_and_zero(i32 %x, i32 %y) {
 ; CHECK-LABEL: define i8 @scmp_of_sub_and_zero(
 ; CHECK-SAME: i32 [[X:%.*]], i32 [[Y:%.*]]) {
-; CHECK-NEXT:    [[DIFF:%.*]] = sub nsw i32 [[X]], [[Y]]
-; CHECK-NEXT:    [[R:%.*]] = call i8 @llvm.scmp.i8.i32(i32 [[DIFF]], i32 0)
+; CHECK-NEXT:    [[R:%.*]] = call i8 @llvm.scmp.i8.i32(i32 [[X]], i32 [[Y]])
 ; CHECK-NEXT:    ret i8 [[R]]
 ;
   %diff = sub nsw i32 %x, %y