[PATCH] D59071: [Transform] Improve saddo with mixed signs

[Dan Robertson via Phabricator via llvm-commits]

dlrobertson created this revision.
dlrobertson added reviewers: nikic, spatel.
Herald added subscribers: llvm-commits, hiraditya.
Herald added a project: LLVM.

Improve folding of the `sadd.with.overflow` intrinsic with `add nsw`
when given constants of mixed signs.


https://reviews.llvm.org/D59071

Files:
  llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
  llvm/test/Transforms/InstCombine/sadd-with-overflow.ll


Index: llvm/test/Transforms/InstCombine/sadd-with-overflow.ll
===================================================================
--- llvm/test/Transforms/InstCombine/sadd-with-overflow.ll
+++ llvm/test/Transforms/InstCombine/sadd-with-overflow.ll
@@ -17,16 +17,27 @@
   ret { i32, i1 } %3
 }
 
-define { i32, i1 } @fold_mixed_signs(i32) {
-; CHECK-LABEL: @fold_mixed_signs(
-; CHECK-NEXT:    [[TMP2:%.*]] = call { i32, i1 } @llvm.sadd.with.overflow.i32(i32 [[TMP0:%.*]], i32 6)
-; CHECK-NEXT:    ret { i32, i1 } [[TMP2]]
+define { i32, i1 } @fold_mixed_signs_first_high(i32) {
+; CHECK-LABEL: @fold_mixed_signs_first_high(
+; CHECK-NEXT:    [[TMP2:%.*]] = add nsw i32 [[TMP0:%.*]], 6
+; CHECK-NEXT:    [[TMP3:%.*]] = insertvalue { i32, i1 } { i32 undef, i1 false }, i32 [[TMP2]], 0
+; CHECK-NEXT:    ret { i32, i1 } [[TMP3]]
 ;
   %2 = add nsw i32 %0, 13
   %3 = tail call { i32, i1 } @llvm.sadd.with.overflow.i32(i32 %2, i32 -7)
   ret { i32, i1 } %3
 }
 
+define { i32, i1 } @fold_mixed_signs_second_high(i32) {
+; CHECK-LABEL: @fold_mixed_signs_second_high(
+; CHECK-NEXT:    [[TMP2:%.*]] = call { i32, i1 } @llvm.sadd.with.overflow.i32(i32 [[TMP0:%.*]], i32 -6)
+; CHECK-NEXT:    ret { i32, i1 } [[TMP2]]
+;
+  %2 = add nsw i32 %0, 7
+  %3 = tail call { i32, i1 } @llvm.sadd.with.overflow.i32(i32 %2, i32 -13)
+  ret { i32, i1 } %3
+}
+
 define { i8, i1 } @fold_on_constant_add_no_overflow(i8) {
 ; CHECK-LABEL: @fold_on_constant_add_no_overflow(
 ; CHECK-NEXT:    [[TMP2:%.*]] = call { i8, i1 } @llvm.sadd.with.overflow.i8(i8 [[TMP0:%.*]], i8 127)
Index: llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
===================================================================
--- llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
+++ llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
@@ -2035,8 +2035,19 @@
     if (Instruction *I = foldIntrinsicWithOverflowCommon(II))
       return I;
 
-    // Given 2 constant operands whose sum does not overflow:
-    // saddo (X +nsw C0), C1 -> saddo X, C0 + C1
+    // Given 2 constant operands whose sum does not overflow the intrinsic
+    // may be folded.
+    //
+    // Cases for `saddo (X +nsw C0), C1 -> saddo X, C0 + C1`:
+    //
+    // - Same sign can be folded to a single `saddo` call.
+    // - Opposite signs with `|C1| > |C0|` can be folded to a single `saddo`
+    //   call.
+    //
+    // Cases for `saddo (X +nsw C0), C1 -> X +nsw (C0 + C1), false`:
+    //
+    // - Opposite signs with `|C1| <= |C0|` can be folded to a single `+nsw`
+    //   call with the overflow flag set to false.
     Value *X;
     const APInt *C0, *C1;
     Value *Arg0 = II->getArgOperand(0);
@@ -2045,11 +2056,19 @@
         match(Arg1, m_APInt(C1))) {
       bool Overflow;
       APInt NewC = C1->sadd_ov(*C0, Overflow);
-      if (!Overflow)
-        return replaceInstUsesWith(
-            *II, Builder.CreateBinaryIntrinsic(
-                     Intrinsic::sadd_with_overflow, X,
-                     ConstantInt::get(Arg1->getType(), NewC)));
+      if (!Overflow) {
+        if (C0->isNegative() == C1->isNegative() || C0->abs().slt(C1->abs())) {
+          return replaceInstUsesWith(
+              *II, Builder.CreateBinaryIntrinsic(
+                       Intrinsic::sadd_with_overflow, X,
+                       ConstantInt::get(Arg1->getType(), NewC)));
+        } else {
+          return CreateOverflowTuple(
+              II,
+              Builder.CreateNSWAdd(X, ConstantInt::get(Arg1->getType(), NewC)),
+              Builder.getFalse());
+        }
+      }
     }
 
     break;


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