[InstCombine] fold sub of min/max of sub with common operand

  x - max(x - y, 0) --> min(x, y)
  x - min(x - y, 0) --> max(x, y)

https://alive2.llvm.org/ce/z/2YkqFe

issue #55470

Author: rotateright

llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp

@@ -1784,6 +1784,15 @@ static Instruction *foldSubOfMinMax(BinaryOperator &I,
     }
   }
 
+  // sub Op0, smin((sub nsw Op0, Z), 0) --> smax Op0, Z
+  // sub Op0, smax((sub nsw Op0, Z), 0) --> smin Op0, Z
+  if (MinMax->isSigned() && match(Y, m_ZeroInt()) &&
+      match(X, m_NSWSub(m_Specific(Op0), m_Value(Z)))) {
+    Intrinsic::ID InvID = getInverseMinMaxIntrinsic(MinMax->getIntrinsicID());
+    Function *F = Intrinsic::getDeclaration(I.getModule(), InvID, Ty);
+    return CallInst::Create(F, {Op0, Z});
+  }
+
   return nullptr;
 }
 

llvm/test/Transforms/InstCombine/sub-minmax.ll

@@ -882,9 +882,7 @@ define i8 @sub_add_umin_use_m(i8 %x, i8 %y, i8 %z) {
 define <2 x i8> @sub_smax0_sub_nsw(<2 x i8> %x, <2 x i8> %y) {
 ; CHECK-LABEL: define {{[^@]+}}@sub_smax0_sub_nsw
 ; CHECK-SAME: (<2 x i8> [[X:%.*]], <2 x i8> [[Y:%.*]]) {
-; CHECK-NEXT:    [[SUB:%.*]] = sub nsw <2 x i8> [[X]], [[Y]]
-; CHECK-NEXT:    [[M:%.*]] = call <2 x i8> @llvm.smax.v2i8(<2 x i8> [[SUB]], <2 x i8> <i8 0, i8 poison>)
-; CHECK-NEXT:    [[R:%.*]] = sub <2 x i8> [[X]], [[M]]
+; CHECK-NEXT:    [[R:%.*]] = call <2 x i8> @llvm.smin.v2i8(<2 x i8> [[X]], <2 x i8> [[Y]])
 ; CHECK-NEXT:    ret <2 x i8> [[R]]
 ;
   %sub = sub nsw <2 x i8> %x, %y
@@ -899,7 +897,7 @@ define i8 @sub_smax0_sub_nsw_use(i8 %x, i8 %y) {
 ; CHECK-NEXT:    [[SUB:%.*]] = sub nsw i8 [[X]], [[Y]]
 ; CHECK-NEXT:    [[M:%.*]] = call i8 @llvm.smax.i8(i8 [[SUB]], i8 0)
 ; CHECK-NEXT:    call void @use8(i8 [[M]])
-; CHECK-NEXT:    [[R:%.*]] = sub i8 [[X]], [[M]]
+; CHECK-NEXT:    [[R:%.*]] = call i8 @llvm.smin.i8(i8 [[X]], i8 [[Y]])
 ; CHECK-NEXT:    ret i8 [[R]]
 ;
   %sub = sub nsw i8 %x, %y
@@ -909,6 +907,8 @@ define i8 @sub_smax0_sub_nsw_use(i8 %x, i8 %y) {
   ret i8 %r
 }
 
+; negative test - must have nsw
+
 define i8 @sub_smax0_sub(i8 %x, i8 %y) {
 ; CHECK-LABEL: define {{[^@]+}}@sub_smax0_sub
 ; CHECK-SAME: (i8 [[X:%.*]], i8 [[Y:%.*]]) {
@@ -923,15 +923,17 @@ define i8 @sub_smax0_sub(i8 %x, i8 %y) {
   ret i8 %r
 }
 
+; negative test - wrong op
+
 define i8 @sub_smax0_sub_commute(i8 %x, i8 %y) {
 ; CHECK-LABEL: define {{[^@]+}}@sub_smax0_sub_commute
 ; CHECK-SAME: (i8 [[X:%.*]], i8 [[Y:%.*]]) {
-; CHECK-NEXT:    [[SUB:%.*]] = sub i8 [[X]], [[Y]]
+; CHECK-NEXT:    [[SUB:%.*]] = sub nsw i8 [[X]], [[Y]]
 ; CHECK-NEXT:    [[M:%.*]] = call i8 @llvm.smax.i8(i8 [[SUB]], i8 0)
 ; CHECK-NEXT:    [[R:%.*]] = sub i8 [[M]], [[X]]
 ; CHECK-NEXT:    ret i8 [[R]]
 ;
-  %sub = sub i8 %x, %y
+  %sub = sub nsw i8 %x, %y
   %m = call i8 @llvm.smax.i8(i8 %sub, i8 0)
   %r = sub i8 %m, %x
   ret i8 %r
@@ -942,8 +944,7 @@ define i8 @sub_smin0_sub_nsw_use(i8 %x, i8 %y) {
 ; CHECK-SAME: (i8 [[X:%.*]], i8 [[Y:%.*]]) {
 ; CHECK-NEXT:    [[SUB:%.*]] = sub nsw i8 [[X]], [[Y]]
 ; CHECK-NEXT:    call void @use8(i8 [[SUB]])
-; CHECK-NEXT:    [[M:%.*]] = call i8 @llvm.smin.i8(i8 [[SUB]], i8 0)
-; CHECK-NEXT:    [[R:%.*]] = sub i8 [[X]], [[M]]
+; CHECK-NEXT:    [[R:%.*]] = call i8 @llvm.smax.i8(i8 [[X]], i8 [[Y]])
 ; CHECK-NEXT:    ret i8 [[R]]
 ;
   %sub = sub nsw i8 %x, %y
@@ -956,9 +957,7 @@ define i8 @sub_smin0_sub_nsw_use(i8 %x, i8 %y) {
 define <2 x i8> @sub_smin0_sub_nsw(<2 x i8> %x, <2 x i8> %y) {
 ; CHECK-LABEL: define {{[^@]+}}@sub_smin0_sub_nsw
 ; CHECK-SAME: (<2 x i8> [[X:%.*]], <2 x i8> [[Y:%.*]]) {
-; CHECK-NEXT:    [[SUB:%.*]] = sub nsw <2 x i8> [[X]], [[Y]]
-; CHECK-NEXT:    [[M:%.*]] = call <2 x i8> @llvm.smin.v2i8(<2 x i8> [[SUB]], <2 x i8> zeroinitializer)
-; CHECK-NEXT:    [[R:%.*]] = sub <2 x i8> [[X]], [[M]]
+; CHECK-NEXT:    [[R:%.*]] = call <2 x i8> @llvm.smax.v2i8(<2 x i8> [[X]], <2 x i8> [[Y]])
 ; CHECK-NEXT:    ret <2 x i8> [[R]]
 ;
   %sub = sub nsw <2 x i8> %x, %y
@@ -967,6 +966,8 @@ define <2 x i8> @sub_smin0_sub_nsw(<2 x i8> %x, <2 x i8> %y) {
   ret <2 x i8> %r
 }
 
+; negative test - must have nsw
+
 define i8 @sub_smin0_sub(i8 %x, i8 %y) {
 ; CHECK-LABEL: define {{[^@]+}}@sub_smin0_sub
 ; CHECK-SAME: (i8 [[X:%.*]], i8 [[Y:%.*]]) {
@@ -981,6 +982,8 @@ define i8 @sub_smin0_sub(i8 %x, i8 %y) {
   ret i8 %r
 }
 
+; negative test - wrong op
+
 define i8 @sub_smin0_sub_nsw_commute(i8 %x, i8 %y) {
 ; CHECK-LABEL: define {{[^@]+}}@sub_smin0_sub_nsw_commute
 ; CHECK-SAME: (i8 [[X:%.*]], i8 [[Y:%.*]]) {