[llvm] [ValueTracking] Fold max/min when incrementing/decrementing by 1 (PR #142466)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-analysis

Author: Alex MacLean (AlexMaclean)

<details>
<summary>Changes</summary>

Add the following folds for integer min max folding in ValueTracking:
 -  (X > Y) ? X : (Y - 1) ==> MIN(X, Y - 1)
 -  (X < Y) ? X : (Y + 1) ==> MAX(X, Y + 1)

These are safe when overflow corresponding to the sign of the comparison is poison. (proof https://alive2.llvm.org/ce/z/oj5iiI).

The most common of these patterns is likely the minimum case which occurs in some internal library code when clamping an integer index to a range (The maximum cases are included for completeness). Here is a simplified example:

```cpp
int clampToWidth(int idx, int width) {
  if (idx >= width)
    return width - 1;
  return idx;
}
```
https://cuda.godbolt.org/z/nhPzWrc3W

---
Full diff: https://github.com/llvm/llvm-project/pull/142466.diff


2 Files Affected:

- (modified) llvm/lib/Analysis/ValueTracking.cpp (+18) 
- (modified) llvm/test/Transforms/InstCombine/minmax-fold.ll (+37) 


``````````diff
diff --git a/llvm/lib/Analysis/ValueTracking.cpp b/llvm/lib/Analysis/ValueTracking.cpp
index fc19b2ccf7964..416d586d52963 100644
--- a/llvm/lib/Analysis/ValueTracking.cpp
+++ b/llvm/lib/Analysis/ValueTracking.cpp
@@ -8388,6 +8388,24 @@ static SelectPatternResult matchMinMax(CmpInst::Predicate Pred,
     }
   }
 
+  // (X > Y) ? X : (Y - 1) ==> MIN(X, Y - 1)
+  // (X < Y) ? X : (Y + 1) ==> MAX(X, Y + 1)
+  // When overflow corresponding to the sign of the comparison is poison.
+  // Note that the UMIN case is not possible as we canonicalize to addition.
+  if (CmpLHS == TrueVal) {
+    if (Pred == CmpInst::ICMP_SGT &&
+        match(FalseVal, m_NSWAddLike(m_Specific(CmpRHS), m_ConstantInt<1>())))
+      return {SPF_SMAX, SPNB_NA, false};
+
+    if (Pred == CmpInst::ICMP_SLT &&
+        match(FalseVal, m_NSWAddLike(m_Specific(CmpRHS), m_ConstantInt<-1>())))
+      return {SPF_SMIN, SPNB_NA, false};
+
+    if (Pred == CmpInst::ICMP_UGT &&
+        match(FalseVal, m_NUWAddLike(m_Specific(CmpRHS), m_ConstantInt<1>())))
+      return {SPF_UMAX, SPNB_NA, false};
+  }
+
   if (Pred != CmpInst::ICMP_SGT && Pred != CmpInst::ICMP_SLT)
     return {SPF_UNKNOWN, SPNB_NA, false};
 
diff --git a/llvm/test/Transforms/InstCombine/minmax-fold.ll b/llvm/test/Transforms/InstCombine/minmax-fold.ll
index cd376b74fb36c..cf3515614321d 100644
--- a/llvm/test/Transforms/InstCombine/minmax-fold.ll
+++ b/llvm/test/Transforms/InstCombine/minmax-fold.ll
@@ -1596,3 +1596,40 @@ define <2 x i32> @test_umax_smax_vec_neg(<2 x i32> %x) {
   %umax = call <2 x i32> @llvm.umax.v2i32(<2 x i32> %smax, <2 x i32> <i32 1, i32 10>)
   ret <2 x i32> %umax
 }
+
+define i32 @test_smin_sub1_nsw(i32 %x, i32 %w) {
+; CHECK-LABEL: @test_smin_sub1_nsw(
+; CHECK-NEXT:    [[SUB:%.*]] = add nsw i32 [[W:%.*]], -1
+; CHECK-NEXT:    [[R:%.*]] = call i32 @llvm.smin.i32(i32 [[X:%.*]], i32 [[SUB]])
+; CHECK-NEXT:    ret i32 [[R]]
+;
+  %cmp = icmp slt i32 %x, %w
+  %sub = add nsw i32 %w, -1
+  %r = select i1 %cmp, i32 %x, i32 %sub
+  ret i32 %r
+}
+
+define i32 @test_smax_add1_nsw(i32 %x, i32 %w) {
+; CHECK-LABEL: @test_smax_add1_nsw(
+; CHECK-NEXT:    [[X2:%.*]] = add nsw i32 [[W:%.*]], 1
+; CHECK-NEXT:    [[R:%.*]] = call i32 @llvm.smax.i32(i32 [[X:%.*]], i32 [[X2]])
+; CHECK-NEXT:    ret i32 [[R]]
+;
+  %cmp = icmp sgt i32 %x, %w
+  %add = add nsw i32 %w, 1
+  %r = select i1 %cmp, i32 %x, i32 %add
+  ret i32 %r
+}
+
+define i32 @test_umax_add1_nsw(i32 %x, i32 %w) {
+; CHECK-LABEL: @test_umax_add1_nsw(
+; CHECK-NEXT:    [[X2:%.*]] = add nuw i32 [[W:%.*]], 1
+; CHECK-NEXT:    [[R:%.*]] = call i32 @llvm.umax.i32(i32 [[X:%.*]], i32 [[X2]])
+; CHECK-NEXT:    ret i32 [[R]]
+;
+  %cmp = icmp ugt i32 %x, %w
+  %add = add nuw i32 %w, 1
+  %r = select i1 %cmp, i32 %x, i32 %add
+  ret i32 %r
+}
+

``````````

</details>


https://github.com/llvm/llvm-project/pull/142466