From 6777ec9e4df79d88f179593e20d6cb58a3effba4 Mon Sep 17 00:00:00 2001
From: Nikita Popov <npopov@redhat.com>
Date: Wed, 23 Feb 2022 12:40:31 +0100
Subject: [PATCH] [ValueTracking] Support signed intrinsic clamp

This is the same special logic we apply for SPF signed clamps
when computing the number of sign bits, just for intrinsics.

This just uses the same logic as the select case, but there's
multiple directions this could be improved in: We could also use
the num sign bits from the clamped value, we could do this during
constant range calculation, and there's probably unsigned analogues
for the constant range case at least.
---
 llvm/lib/Analysis/ValueTracking.cpp                | 24 ++++++++++++++++++++++
 llvm/test/Transforms/InstCombine/max_known_bits.ll |  8 +++-----
 2 files changed, 27 insertions(+), 5 deletions(-)

diff --git a/llvm/lib/Analysis/ValueTracking.cpp b/llvm/lib/Analysis/ValueTracking.cpp
index ef84e0c..b8ff6be 100644
--- a/llvm/lib/Analysis/ValueTracking.cpp
+++ b/llvm/lib/Analysis/ValueTracking.cpp
@@ -2902,6 +2902,24 @@ static bool isSignedMinMaxClamp(const Value *Select, const Value *&In,
   return CLow->sle(*CHigh);
 }
 
+static bool isSignedMinMaxIntrinsicClamp(const IntrinsicInst *II,
+                                         const APInt *&CLow,
+                                         const APInt *&CHigh) {
+  assert((II->getIntrinsicID() == Intrinsic::smin ||
+          II->getIntrinsicID() == Intrinsic::smax) && "Must be smin/smax");
+
+  Intrinsic::ID InverseID = getInverseMinMaxIntrinsic(II->getIntrinsicID());
+  auto *InnerII = dyn_cast<IntrinsicInst>(II->getArgOperand(0));
+  if (!InnerII || InnerII->getIntrinsicID() != InverseID ||
+      !match(II->getArgOperand(1), m_APInt(CLow)) ||
+      !match(InnerII->getArgOperand(1), m_APInt(CHigh)))
+    return false;
+
+  if (II->getIntrinsicID() == Intrinsic::smin)
+    std::swap(CLow, CHigh);
+  return CLow->sle(*CHigh);
+}
+
 /// For vector constants, loop over the elements and find the constant with the
 /// minimum number of sign bits. Return 0 if the value is not a vector constant
 /// or if any element was not analyzed; otherwise, return the count for the
@@ -3242,6 +3260,12 @@ static unsigned ComputeNumSignBitsImpl(const Value *V,
 
           // Absolute value reduces number of sign bits by at most 1.
           return Tmp - 1;
+        case Intrinsic::smin:
+        case Intrinsic::smax: {
+          const APInt *CLow, *CHigh;
+          if (isSignedMinMaxIntrinsicClamp(II, CLow, CHigh))
+            return std::min(CLow->getNumSignBits(), CHigh->getNumSignBits());
+        }
         }
       }
     }
diff --git a/llvm/test/Transforms/InstCombine/max_known_bits.ll b/llvm/test/Transforms/InstCombine/max_known_bits.ll
index a9f5795..7ae179d 100644
--- a/llvm/test/Transforms/InstCombine/max_known_bits.ll
+++ b/llvm/test/Transforms/InstCombine/max_known_bits.ll
@@ -116,7 +116,7 @@ define i16 @min_max_clamp_intrinsic_2(i16 %x) {
 ; CHECK-LABEL: @min_max_clamp_intrinsic_2(
 ; CHECK-NEXT:    [[A:%.*]] = call i16 @llvm.smin.i16(i16 [[X:%.*]], i16 2047)
 ; CHECK-NEXT:    [[B:%.*]] = call i16 @llvm.smax.i16(i16 [[A]], i16 -2048)
-; CHECK-NEXT:    [[C:%.*]] = add i16 [[B]], 1
+; CHECK-NEXT:    [[C:%.*]] = add nsw i16 [[B]], 1
 ; CHECK-NEXT:    ret i16 [[C]]
 ;
   %a = call i16 @llvm.smin.i16(i16 %x, i16 2047)
@@ -144,10 +144,8 @@ define i32 @min_max_clamp_intrinsic_4(i16 %x) {
 ; CHECK-LABEL: @min_max_clamp_intrinsic_4(
 ; CHECK-NEXT:    [[A:%.*]] = call i16 @llvm.smin.i16(i16 [[X:%.*]], i16 2047)
 ; CHECK-NEXT:    [[B:%.*]] = call i16 @llvm.smax.i16(i16 [[A]], i16 -2048)
-; CHECK-NEXT:    [[C:%.*]] = add i16 [[B]], 1
-; CHECK-NEXT:    [[D:%.*]] = sext i16 [[C]] to i32
-; CHECK-NEXT:    [[E:%.*]] = add nsw i32 [[D]], -1
-; CHECK-NEXT:    ret i32 [[E]]
+; CHECK-NEXT:    [[TMP1:%.*]] = sext i16 [[B]] to i32
+; CHECK-NEXT:    ret i32 [[TMP1]]
 ;
   %a = call i16 @llvm.smin.i16(i16 %x, i16 2047)
   %b = call i16 @llvm.smax.i16(i16 %a, i16 -2048)
-- 
2.7.4