[PATCH] D42262: [JumpThreading] Don't restrict cast-traversal to i1

[Keno Fischer via Phabricator via llvm-commits]

loladiro created this revision.
loladiro added reviewers: haicheng, gberry, bmakam, mcrosier.

In https://reviews.llvm.org/D17663, JumpThreading learned to look trough simple cast instructions,
but only if the source of those cast instructions was a phi/cmp i1
(in an effort to limit compile time effects). I think this condition
is too restrictive. For switches with limited value range, InstCombine
will readily introduce an extra `trunc` instruction to a smaller
integer type (e.g. from i8 to i2), leaving us in the somewhat perverse
situation that jump-threading would work before running instcombine,
but not after. Since instcombine produces this pattern, I think we
need to consider it canonical and support it in JumpThreading.
In general, for limiting recursion, I think the existing restriction
to phi and cmp nodes should be sufficient to avoid looking through
unprofitable chains of instructions.


Repository:
  rL LLVM

https://reviews.llvm.org/D42262

Files:
  lib/Transforms/Scalar/JumpThreading.cpp
  test/Transforms/JumpThreading/basic.ll


Index: test/Transforms/JumpThreading/basic.ll
===================================================================
--- test/Transforms/JumpThreading/basic.ll
+++ test/Transforms/JumpThreading/basic.ll
@@ -547,6 +547,34 @@
 ; CHECK: }
 }
 
+define i1 @trunc_switch(i1 %arg) {
+; CHECK-LABEL: @trunc_switch
+top:
+; CHECK: br i1 %arg, label %exitA, label %exitB
+  br i1 %arg, label %common, label %B
+
+B:
+  br label %common
+
+common:
+  %phi = phi i8 [ 2, %B ], [ 1, %top ]
+  %trunc = trunc i8 %phi to i2
+; CHECK-NOT: switch
+  switch i2 %trunc, label %unreach [
+    i2 1, label %exitA
+    i2 -2, label %exitB
+  ]
+
+unreach:
+  unreachable
+
+exitA:
+  ret i1 true
+
+exitB:
+  ret i1 false
+}
+
 ; CHECK-LABEL: define void @h_con(i32 %p) {
 define void @h_con(i32 %p) {
   %x = icmp ult i32 %p, 5
Index: lib/Transforms/Scalar/JumpThreading.cpp
===================================================================
--- lib/Transforms/Scalar/JumpThreading.cpp
+++ lib/Transforms/Scalar/JumpThreading.cpp
@@ -647,11 +647,9 @@
   }
 
   // Handle Cast instructions.  Only see through Cast when the source operand is
-  // PHI or Cmp and the source type is i1 to save the compilation time.
+  // PHI or Cmp to save the compilation time.
   if (CastInst *CI = dyn_cast<CastInst>(I)) {
     Value *Source = CI->getOperand(0);
-    if (!Source->getType()->isIntegerTy(1))
-      return false;
     if (!isa<PHINode>(Source) && !isa<CmpInst>(Source))
       return false;
     ComputeValueKnownInPredecessors(Source, BB, Result, Preference, CxtI);


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