From de3fa35cdb6f5136b34716396763b15ed01d186b Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?D=C3=A1vid=20Bolvansk=C3=BD?= <david.bolvansky@gmail.com>
Date: Sat, 24 Apr 2021 18:24:54 +0200
Subject: [PATCH] [InstCombine] ctpop(rot(X)) -> ctpop(X)

Proof:
https://alive2.llvm.org/ce/z/ss2zyt - rotl
https://alive2.llvm.org/ce/z/ZM7Aue - rotr

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D101235
---
 .../InstCombine/InstCombineCalls.cpp          |  5 ++++
 llvm/test/Transforms/InstCombine/ctpop.ll     | 27 +++++++++++++++++--
 2 files changed, 30 insertions(+), 2 deletions(-)

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
index 954c853ab7f0..378682b7b9cd 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
@@ -504,6 +504,11 @@ static Instruction *foldCtpop(IntrinsicInst &II, InstCombinerImpl &IC) {
   if (match(Op0, m_BitReverse(m_Value(X))) || match(Op0, m_BSwap(m_Value(X))))
     return IC.replaceOperand(II, 0, X);
 
+  // ctpop(rot(x)) -> ctpop(x)
+  if (match(Op0, m_FShl(m_Value(X), m_Specific(X), m_Value())) ||
+      match(Op0, m_FShr(m_Value(X), m_Specific(X), m_Value())))
+    return IC.replaceOperand(II, 0, X);
+
   // ctpop(x | -x) -> bitwidth - cttz(x, false)
   if (Op0->hasOneUse() &&
       match(Op0, m_c_Or(m_Value(X), m_Neg(m_Deferred(X))))) {
diff --git a/llvm/test/Transforms/InstCombine/ctpop.ll b/llvm/test/Transforms/InstCombine/ctpop.ll
index 9a238d27fd31..b389b1f6617a 100644
--- a/llvm/test/Transforms/InstCombine/ctpop.ll
+++ b/llvm/test/Transforms/InstCombine/ctpop.ll
@@ -203,11 +203,11 @@ define i32 @ctpop_add(i32 %a, i32 %b) {
 
 define i32 @ctpop_add_no_common_bits(i32 %a, i32 %b) {
 ; CHECK-LABEL: @ctpop_add_no_common_bits(
-; CHECK-NEXT:    [[TMP1:%.*]] = call i32 @llvm.fshl.i32(i32 [[B:%.*]], i32 [[B]], i32 16)
+; CHECK-NEXT:    [[TMP1:%.*]] = call i32 @llvm.fshl.i32(i32 [[A:%.*]], i32 [[B:%.*]], i32 16)
 ; CHECK-NEXT:    [[TMP2:%.*]] = call i32 @llvm.ctpop.i32(i32 [[TMP1]]), !range [[RNG1]]
 ; CHECK-NEXT:    ret i32 [[TMP2]]
 ;
-  %shl16 = shl i32 %b, 16
+  %shl16 = shl i32 %a, 16
   %ctpop1 = tail call i32 @llvm.ctpop.i32(i32 %shl16)
   %lshl16 = lshr i32 %b, 16
   %ctpop2 = tail call i32 @llvm.ctpop.i32(i32 %lshl16)
@@ -266,3 +266,26 @@ define <2 x i32> @ctpop_add_no_common_bits_vec_use2(<2 x i32> %a, <2 x i32> %b,
   %res = add <2 x i32> %ctpop1, %ctpop2
   ret <2 x i32> %res
 }
+
+define i8 @ctpop_rotate_left(i8 %a, i8 %amt)  {
+; CHECK-LABEL: @ctpop_rotate_left(
+; CHECK-NEXT:    [[CTPOP:%.*]] = tail call i8 @llvm.ctpop.i8(i8 [[A:%.*]]), !range [[RNG0]]
+; CHECK-NEXT:    ret i8 [[CTPOP]]
+;
+  %rotl = tail call i8 @llvm.fshl.i8(i8 %a, i8 %a, i8 %amt)
+  %ctpop = tail call i8 @llvm.ctpop.i8(i8 %rotl)
+  ret i8 %ctpop
+}
+
+define i8 @ctpop_rotate_right(i8 %a, i8 %amt)  {
+; CHECK-LABEL: @ctpop_rotate_right(
+; CHECK-NEXT:    [[CTPOP:%.*]] = tail call i8 @llvm.ctpop.i8(i8 [[A:%.*]]), !range [[RNG0]]
+; CHECK-NEXT:    ret i8 [[CTPOP]]
+;
+  %rotr = tail call i8 @llvm.fshr.i8(i8 %a, i8 %a, i8 %amt)
+  %ctpop = tail call i8 @llvm.ctpop.i8(i8 %rotr)
+  ret i8 %ctpop
+}
+
+declare i8 @llvm.fshl.i8(i8, i8, i8)
+declare i8 @llvm.fshr.i8(i8, i8, i8)