[llvm] r268534 - [ConstantFold] Don't try to strip fp -> int bitcasts to simplify icmps

[Hal Finkel via llvm-commits]

Author: hfinkel
Date: Wed May  4 14:37:08 2016
New Revision: 268534

URL: http://llvm.org/viewvc/llvm-project?rev=268534&view=rev
Log:
[ConstantFold] Don't try to strip fp -> int bitcasts to simplify icmps

ConstantFold has logic to take icmp (bitcast x to y), null and strip the
bitcast. This makes sense in general, but not if x has floating-point type. In
this case, we'd need a fcmp, not an icmp, and the code will assert. We normally
don't see this situation because we constant fold fp -> int bitcasts, however,
we'll see it for bitcasts of ppc_fp128 -> i128. This is because that bitcast is
Endian-dependent, and as a result, we don't simplify it in ConstantFold (we
could, but no one has yet added the necessary logic). Regardless, ConstantFold
should not depend on that canonicalization for correctness.

Added:
    llvm/trunk/test/Transforms/IPConstantProp/fp-bc-icmp-const-fold.ll
Modified:
    llvm/trunk/lib/IR/ConstantFold.cpp

Modified: llvm/trunk/lib/IR/ConstantFold.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/IR/ConstantFold.cpp?rev=268534&r1=268533&r2=268534&view=diff
==============================================================================
--- llvm/trunk/lib/IR/ConstantFold.cpp (original)
+++ llvm/trunk/lib/IR/ConstantFold.cpp Wed May  4 14:37:08 2016
@@ -1529,6 +1529,10 @@ static ICmpInst::Predicate evaluateICmpR
     case Instruction::BitCast:
     case Instruction::ZExt:
     case Instruction::SExt:
+      // We can't evaluate floating point casts or truncations.
+      if (CE1Op0->getType()->isFloatingPointTy())
+        break;
+
       // If the cast is not actually changing bits, and the second operand is a
       // null pointer, do the comparison with the pre-casted value.
       if (V2->isNullValue() &&

Added: llvm/trunk/test/Transforms/IPConstantProp/fp-bc-icmp-const-fold.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/IPConstantProp/fp-bc-icmp-const-fold.ll?rev=268534&view=auto
==============================================================================
--- llvm/trunk/test/Transforms/IPConstantProp/fp-bc-icmp-const-fold.ll (added)
+++ llvm/trunk/test/Transforms/IPConstantProp/fp-bc-icmp-const-fold.ll Wed May  4 14:37:08 2016
@@ -0,0 +1,52 @@
+; RUN: opt -S -ipsccp < %s | FileCheck %s
+target datalayout = "E-m:e-i64:64-n32:64"
+target triple = "powerpc64-bgq-linux"
+
+define void @test(i32 signext %n) {
+
+; CHECK-LABEL: @test
+
+entry:
+  br i1 undef, label %if.then, label %if.end
+
+if.then:                                          ; preds = %entry
+  ret void
+
+if.end:                                           ; preds = %entry
+  br i1 undef, label %if.then2, label %if.end4
+
+if.then2:                                         ; preds = %if.end
+  unreachable
+
+if.end4:                                          ; preds = %if.end
+  %sub.n = select i1 undef, i32 undef, i32 %n
+  switch i32 %sub.n, label %if.else14 [
+    i32 0, label %if.then9
+    i32 1, label %if.then12
+  ]
+
+if.then9:                                         ; preds = %if.end4
+  unreachable
+
+if.then12:                                        ; preds = %if.end4
+  unreachable
+
+if.else14:                                        ; preds = %if.end4
+  br label %do.body
+
+do.body:                                          ; preds = %do.body, %if.else14
+  %scale.0 = phi ppc_fp128 [ 0xM3FF00000000000000000000000000000, %if.else14 ], [ %scale.0, %do.body ]
+  br i1 undef, label %do.body, label %if.then33
+
+if.then33:                                        ; preds = %do.body
+  br i1 undef, label %_ZN5boost4math4signIgEEiRKT_.exit30, label %cond.false.i28
+
+cond.false.i28:                                   ; preds = %if.then33
+  %0 = bitcast ppc_fp128 %scale.0 to i128
+  %tobool.i26 = icmp slt i128 %0, 0
+  br label %_ZN5boost4math4signIgEEiRKT_.exit30
+
+_ZN5boost4math4signIgEEiRKT_.exit30:              ; preds = %cond.false.i28, %if.then33
+  unreachable
+}
+