[X86][fast-isel] Fix select lowering.

The condition in selects is supposed to be i1.
Make sure we are just reading the less significant bit
of the 8 bits width value to match this constraint.

<rdar://problem/15651765>

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@197712 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/lib/Target/X86/X86FastISel.cpp b/lib/Target/X86/X86FastISel.cpp
index 972b82fa9463006309f8b5c478706c9f188662c6..7be2a14a44f229871031a46b0c7f0cd5cae88bdc 100644 (file)
--- a/lib/Target/X86/X86FastISel.cpp
+++ b/lib/Target/X86/X86FastISel.cpp
@@ -1508,8 +1508,13 @@ bool X86FastISel::X86SelectSelect(const Instruction *I) {
   unsigned Op2Reg = getRegForValue(I->getOperand(2));
   if (Op2Reg == 0) return false;
 
-  BuildMI(*FuncInfo.MBB, FuncInfo.InsertPt, DL, TII.get(X86::TEST8rr))
-    .addReg(Op0Reg).addReg(Op0Reg);
+  // Selects operate on i1, however, Op0Reg is 8 bits width and may contain
+  // garbage. Indeed, only the less significant bit is supposed to be accurate.
+  // If we read more than the lsb, we may see non-zero values whereas lsb
+  // is zero. Therefore, we have to truncate Op0Reg to i1 for the select.
+  // This is acheived by performing TEST against 1.
+  BuildMI(*FuncInfo.MBB, FuncInfo.InsertPt, DL, TII.get(X86::TEST8ri))
+    .addReg(Op0Reg).addImm(1);
   unsigned ResultReg = createResultReg(RC);
   BuildMI(*FuncInfo.MBB, FuncInfo.InsertPt, DL, TII.get(Opc), ResultReg)
     .addReg(Op1Reg).addReg(Op2Reg);

diff --git a/test/CodeGen/X86/fast-isel-select.ll b/test/CodeGen/X86/fast-isel-select.ll
new file mode 100644 (file)
index 0000000..53158bc
--- /dev/null
+++ b/test/CodeGen/X86/fast-isel-select.ll
@@ -0,0 +1,16 @@
+; RUN: llc -mtriple x86_64-apple-darwin -O0 -o - < %s | FileCheck %s
+; Make sure we only use the less significant bit of the value that feeds the
+; select. Otherwise, we may account for a non-zero value whereas the
+; lsb is zero.
+; <rdar://problem/15651765>
+
+; CHECK-LABEL: fastisel_select: 
+; CHECK: subb {{%[a-z0-9]+}}, [[RES:%[a-z0-9]+]]
+; CHECK: testb $1, [[RES]]
+; CHECK: cmovel
+define i32 @fastisel_select(i1 %exchSub2211_, i1 %trunc_8766) {
+  %shuffleInternal15257_8932 = sub i1 %exchSub2211_, %trunc_8766
+  %counter_diff1345 = select i1 %shuffleInternal15257_8932, i32 1204476887, i32 0
+  ret i32 %counter_diff1345
+}
+