[PATCH] D47891: [InstSimplify] shl nuw C, %x -> C iff KnownBits says C is negative.

[Roman Lebedev via Phabricator via llvm-commits]

lebedev.ri created this revision.
lebedev.ri added reviewers: spatel, craig.topper.

Follow-up for https://reviews.llvm.org/D47883.


Repository:
  rL LLVM

https://reviews.llvm.org/D47891

Files:
  lib/Analysis/InstructionSimplify.cpp
  test/Transforms/InstCombine/select-obo-peo-ops.ll
  test/Transforms/InstSimplify/constantfold-shl-nuw-C-to-C.ll


Index: test/Transforms/InstSimplify/constantfold-shl-nuw-C-to-C.ll
===================================================================
--- test/Transforms/InstSimplify/constantfold-shl-nuw-C-to-C.ll
+++ test/Transforms/InstSimplify/constantfold-shl-nuw-C-to-C.ll
@@ -44,8 +44,7 @@
 ; CHECK-LABEL: @knownbits_negative(
 ; CHECK-NEXT:    [[CMP:%.*]] = icmp slt i8 [[X:%.*]], 0
 ; CHECK-NEXT:    tail call void @llvm.assume(i1 [[CMP]])
-; CHECK-NEXT:    [[RET:%.*]] = shl nuw i8 [[X]], [[Y:%.*]]
-; CHECK-NEXT:    ret i8 [[RET]]
+; CHECK-NEXT:    ret i8 [[X]]
 ;
   %cmp = icmp slt i8 %x, 0
   tail call void @llvm.assume(i1 %cmp)
Index: test/Transforms/InstCombine/select-obo-peo-ops.ll
===================================================================
--- test/Transforms/InstCombine/select-obo-peo-ops.ll
+++ test/Transforms/InstCombine/select-obo-peo-ops.ll
@@ -157,11 +157,10 @@
 ; CHECK-LABEL: @test_shl_nuw_nsw__nsw_is_safe(
 ; CHECK-NEXT:    [[TMP1:%.*]] = or i32 [[X:%.*]], -83886080
 ; CHECK-NEXT:    [[TMP2:%.*]] = icmp eq i32 [[TMP1]], -83886079
-; CHECK-NEXT:    [[TMP3:%.*]] = shl nuw nsw i32 [[TMP1]], 2
-; CHECK-NEXT:    [[TMP4:%.*]] = select i1 [[TMP2]], i32 -335544316, i32 [[TMP3]]
+; CHECK-NEXT:    [[TMP3:%.*]] = select i1 [[TMP2]], i32 -335544316, i32 [[TMP1]]
+; CHECK-NEXT:    [[TMP4:%.*]] = mul i32 [[TMP3]], [[TMP1]]
 ; CHECK-NEXT:    [[TMP5:%.*]] = mul i32 [[TMP4]], [[TMP1]]
-; CHECK-NEXT:    [[TMP6:%.*]] = mul i32 [[TMP5]], [[TMP3]]
-; CHECK-NEXT:    ret i32 [[TMP6]]
+; CHECK-NEXT:    ret i32 [[TMP5]]
 ;
   %1 = or i32 %x, -83886080
   %2 = icmp eq i32 %1, -83886079
@@ -176,11 +175,10 @@
 ; CHECK-LABEL: @test_shl_nuw__nsw_is_safe(
 ; CHECK-NEXT:    [[TMP1:%.*]] = or i32 [[X:%.*]], -83886080
 ; CHECK-NEXT:    [[TMP2:%.*]] = icmp eq i32 [[TMP1]], -83886079
-; CHECK-NEXT:    [[TMP3:%.*]] = shl nuw nsw i32 [[TMP1]], 2
-; CHECK-NEXT:    [[TMP4:%.*]] = select i1 [[TMP2]], i32 -335544316, i32 [[TMP3]]
+; CHECK-NEXT:    [[TMP3:%.*]] = select i1 [[TMP2]], i32 -335544316, i32 [[TMP1]]
+; CHECK-NEXT:    [[TMP4:%.*]] = mul i32 [[TMP3]], [[TMP1]]
 ; CHECK-NEXT:    [[TMP5:%.*]] = mul i32 [[TMP4]], [[TMP1]]
-; CHECK-NEXT:    [[TMP6:%.*]] = mul i32 [[TMP5]], [[TMP3]]
-; CHECK-NEXT:    ret i32 [[TMP6]]
+; CHECK-NEXT:    ret i32 [[TMP5]]
 ;
   %1 = or i32 %x, -83886080
   %2 = icmp eq i32 %1, -83886079
Index: lib/Analysis/InstructionSimplify.cpp
===================================================================
--- lib/Analysis/InstructionSimplify.cpp
+++ lib/Analysis/InstructionSimplify.cpp
@@ -1228,8 +1228,19 @@
     return X;
 
   // shl nuw i8 C, %x  ->  C  iff C has sign bit set.
-  if (isNUW && match(Op0, m_Negative()))
-    return Op0;
+  if (isNUW) {
+    if (match(Op0, m_Constant()))
+      return match(Op0, m_Negative()) ? Op0 : nullptr;
+
+    // If Op0 is a constant, then ^ would already have definitively told us
+    // if it is negative, thus no point in going further it it is a constant.
+
+    if (computeKnownBits(Op0, Q.DL, /*Depth=*/0, Q.AC, Q.CxtI, Q.DT)
+            .isNegative())
+      return Op0;
+
+    // NOTE: Using LazyValueInfo could catch more cases.
+  }
 
   return nullptr;
 }


-------------- next part --------------
A non-text attachment was scrubbed...
Name: D47891.150352.patch
Type: text/x-patch
Size: 3152 bytes
Desc: not available
URL: <http://lists.llvm.org/pipermail/llvm-commits/attachments/20180607/da239873/attachment.bin>