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

In r263618, 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.

Patch by Keno Fischer!
Differential Revision: https://reviews.llvm.org/D42262

llvm-svn: 327150

lib/Transforms/Scalar/JumpThreading.cpp

@@ -656,11 +656,9 @@ bool JumpThreadingPass::ComputeValueKnownInPredecessors(
   }
 
   // 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);

test/Transforms/JumpThreading/basic.ll

@@ -547,6 +547,34 @@ l5:
 ; 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