[InstCombine] fold min/max intrinsic with negated operand to abs

The smax case shows up in https://llvm.org/PR49885 .
The others seem unlikely, but we might as well try
for uniformity (although that could mean an extra
instruction to create "nabs").

smax -- https://alive2.llvm.org/ce/z/8yYaGy
smin -- https://alive2.llvm.org/ce/z/0_7zc_
umax -- https://alive2.llvm.org/ce/z/EcsZWs
umin -- https://alive2.llvm.org/ce/z/Xw6WvB

Author: rotateright

llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp

@@ -911,6 +911,22 @@ Instruction *InstCombinerImpl::visitCallInst(CallInst &CI) {
       }
     }
 
+    // smax(X, -X) --> abs(X)
+    // smin(X, -X) --> -abs(X)
+    // umax(X, -X) --> -abs(X)
+    // umin(X, -X) --> abs(X)
+    if (isKnownNegation(I0, I1)) {
+      // This is some variant of abs(). See if we can propagate 'nsw' to the abs
+      // operation and potentially its negation.
+      bool IntMinIsPoison = isKnownNegation(I0, I1, /* NeedNSW */ true);
+      Value *Abs = Builder.CreateBinaryIntrinsic(
+          Intrinsic::abs, I0,
+          ConstantInt::getBool(II->getContext(), IntMinIsPoison));
+      if (IID == Intrinsic::smin || IID == Intrinsic::umax)
+        Abs = Builder.CreateNeg(Abs, "nabs", /* NUW */ false, IntMinIsPoison);
+      return replaceInstUsesWith(CI, Abs);
+    }
+
     break;
   }
   case Intrinsic::bswap: {

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

@@ -611,9 +611,8 @@ define i8 @not_umin_of_not_constant_op(i8 %x) {
 define i8 @smax_negation(i8 %x, i8 %y) {
 ; CHECK-LABEL: @smax_negation(
 ; CHECK-NEXT:    [[S1:%.*]] = sub i8 [[X:%.*]], [[Y:%.*]]
-; CHECK-NEXT:    [[S2:%.*]] = sub i8 [[Y]], [[X]]
-; CHECK-NEXT:    [[R:%.*]] = call i8 @llvm.smax.i8(i8 [[S1]], i8 [[S2]])
-; CHECK-NEXT:    ret i8 [[R]]
+; CHECK-NEXT:    [[TMP1:%.*]] = call i8 @llvm.abs.i8(i8 [[S1]], i1 false)
+; CHECK-NEXT:    ret i8 [[TMP1]]
 ;
   %s1 = sub i8 %x, %y
   %s2 = sub i8 %y, %x
@@ -624,9 +623,8 @@ define i8 @smax_negation(i8 %x, i8 %y) {
 define i8 @smax_negation_nsw(i8 %x, i8 %y) {
 ; CHECK-LABEL: @smax_negation_nsw(
 ; CHECK-NEXT:    [[S1:%.*]] = sub nsw i8 [[X:%.*]], [[Y:%.*]]
-; CHECK-NEXT:    [[S2:%.*]] = sub nsw i8 [[Y]], [[X]]
-; CHECK-NEXT:    [[R:%.*]] = call i8 @llvm.smax.i8(i8 [[S1]], i8 [[S2]])
-; CHECK-NEXT:    ret i8 [[R]]
+; CHECK-NEXT:    [[TMP1:%.*]] = call i8 @llvm.abs.i8(i8 [[S1]], i1 true)
+; CHECK-NEXT:    ret i8 [[TMP1]]
 ;
   %s1 = sub nsw i8 %x, %y
   %s2 = sub nsw i8 %y, %x
@@ -637,9 +635,8 @@ define i8 @smax_negation_nsw(i8 %x, i8 %y) {
 define i8 @smax_negation_not_nsw(i8 %x, i8 %y) {
 ; CHECK-LABEL: @smax_negation_not_nsw(
 ; CHECK-NEXT:    [[S1:%.*]] = sub nsw i8 [[X:%.*]], [[Y:%.*]]
-; CHECK-NEXT:    [[S2:%.*]] = sub nuw i8 [[Y]], [[X]]
-; CHECK-NEXT:    [[R:%.*]] = call i8 @llvm.smax.i8(i8 [[S1]], i8 [[S2]])
-; CHECK-NEXT:    ret i8 [[R]]
+; CHECK-NEXT:    [[TMP1:%.*]] = call i8 @llvm.abs.i8(i8 [[S1]], i1 false)
+; CHECK-NEXT:    ret i8 [[TMP1]]
 ;
   %s1 = sub nsw i8 %x, %y
   %s2 = sub nuw i8 %y, %x
@@ -649,9 +646,8 @@ define i8 @smax_negation_not_nsw(i8 %x, i8 %y) {
 
 define <3 x i8> @smax_negation_vec(<3 x i8> %x) {
 ; CHECK-LABEL: @smax_negation_vec(
-; CHECK-NEXT:    [[S:%.*]] = sub <3 x i8> <i8 0, i8 undef, i8 0>, [[X:%.*]]
-; CHECK-NEXT:    [[R:%.*]] = call <3 x i8> @llvm.smax.v3i8(<3 x i8> [[X]], <3 x i8> [[S]])
-; CHECK-NEXT:    ret <3 x i8> [[R]]
+; CHECK-NEXT:    [[TMP1:%.*]] = call <3 x i8> @llvm.abs.v3i8(<3 x i8> [[X:%.*]], i1 false)
+; CHECK-NEXT:    ret <3 x i8> [[TMP1]]
 ;
   %s = sub <3 x i8> <i8 0, i8 undef, i8 0>, %x
   %r = call <3 x i8> @llvm.smax.v3i8(<3 x i8> %x, <3 x i8> %s)
@@ -661,9 +657,9 @@ define <3 x i8> @smax_negation_vec(<3 x i8> %x) {
 define i8 @smin_negation(i8 %x, i8 %y) {
 ; CHECK-LABEL: @smin_negation(
 ; CHECK-NEXT:    [[S1:%.*]] = sub i8 [[X:%.*]], [[Y:%.*]]
-; CHECK-NEXT:    [[S2:%.*]] = sub i8 [[Y]], [[X]]
-; CHECK-NEXT:    [[R:%.*]] = call i8 @llvm.smin.i8(i8 [[S1]], i8 [[S2]])
-; CHECK-NEXT:    ret i8 [[R]]
+; CHECK-NEXT:    [[TMP1:%.*]] = call i8 @llvm.abs.i8(i8 [[S1]], i1 false)
+; CHECK-NEXT:    [[NABS:%.*]] = sub i8 0, [[TMP1]]
+; CHECK-NEXT:    ret i8 [[NABS]]
 ;
   %s1 = sub i8 %x, %y
   %s2 = sub i8 %y, %x
@@ -674,9 +670,9 @@ define i8 @smin_negation(i8 %x, i8 %y) {
 define i8 @umax_negation(i8 %x, i8 %y) {
 ; CHECK-LABEL: @umax_negation(
 ; CHECK-NEXT:    [[S1:%.*]] = sub nsw i8 [[X:%.*]], [[Y:%.*]]
-; CHECK-NEXT:    [[S2:%.*]] = sub nsw i8 [[Y]], [[X]]
-; CHECK-NEXT:    [[R:%.*]] = call i8 @llvm.umax.i8(i8 [[S1]], i8 [[S2]])
-; CHECK-NEXT:    ret i8 [[R]]
+; CHECK-NEXT:    [[TMP1:%.*]] = call i8 @llvm.abs.i8(i8 [[S1]], i1 true)
+; CHECK-NEXT:    [[NABS:%.*]] = sub nsw i8 0, [[TMP1]]
+; CHECK-NEXT:    ret i8 [[NABS]]
 ;
   %s1 = sub nsw i8 %x, %y
   %s2 = sub nsw i8 %y, %x
@@ -686,9 +682,8 @@ define i8 @umax_negation(i8 %x, i8 %y) {
 
 define i8 @umin_negation(i8 %x) {
 ; CHECK-LABEL: @umin_negation(
-; CHECK-NEXT:    [[S:%.*]] = sub nsw i8 0, [[X:%.*]]
-; CHECK-NEXT:    [[R:%.*]] = call i8 @llvm.umin.i8(i8 [[S]], i8 [[X]])
-; CHECK-NEXT:    ret i8 [[R]]
+; CHECK-NEXT:    [[TMP1:%.*]] = call i8 @llvm.abs.i8(i8 [[X:%.*]], i1 true)
+; CHECK-NEXT:    ret i8 [[TMP1]]
 ;
   %s = sub nsw i8 0, %x
   %r = call i8 @llvm.umin.i8(i8 %s, i8 %x)