Disable shift-compare instcombine optimization for NaCl.

This is one of at least 2 optimizations which introduce non-power-of-two
integer sizes, which we don't want to allow in the PNaCl stable wire format.
This transforms
(icmp pred iM (shl iM %value, N), ConstInt)
  -> (icmp pred i(M-N) (trunc %value iM to i(N-N)), (trunc (ConstInt >> N))
and reduces the size of the compare and its operands.

R=mseaborn@chromium.org
BUG= https://code.google.com/p/nativeclient/issues/detail?id=3360

Review URL: https://codereview.chromium.org/13182003

2 files changed, 30 insertions, 1 deletions

diff --git a/lib/Transforms/InstCombine/InstCombineCompares.cpp b/lib/Transforms/InstCombine/InstCombineCompares.cpp
index bad46b4dab..0267117421 100644
--- a/lib/Transforms/InstCombine/InstCombineCompares.cpp
+++ b/lib/Transforms/InstCombine/InstCombineCompares.cpp
@@ -1339,7 +1339,12 @@ Instruction *InstCombiner::visitICmpInstWithInstAndIntCst(ICmpInst &ICI,
     // free on the target. It has the additional benefit of comparing to a
     // smaller constant, which will be target friendly.
     unsigned Amt = ShAmt->getLimitedValue(TypeBits-1);
-    if (Amt != 0 && RHSV.countTrailingZeros() >= Amt) {
+    // @LOCALMOD-BEGIN
+    // We don't want to introduce non-power-of-two integer sizes for PNaCl's
+    // stable wire format, so modify this transformation for NaCl.
+    if (Amt != 0 && RHSV.countTrailingZeros() >= Amt &&
+        isPowerOf2_32(TypeBits - Amt) && (TypeBits - Amt) >= 8) {
+      // @LOCALMOD-END
       Type *NTy = IntegerType::get(ICI.getContext(), TypeBits - Amt);
       Constant *NCI = ConstantExpr::getTrunc(
                         ConstantExpr::getAShr(RHS,
diff --git a/test/NaCl/PNaClABI/instcombine.ll b/test/NaCl/PNaClABI/instcombine.ll
new file mode 100644
index 0000000000..e21eea45c4
--- /dev/null
+++ b/test/NaCl/PNaClABI/instcombine.ll
@@ -0,0 +1,24 @@
+; RUN: opt < %s -instcombine -S | FileCheck %s
+; Test that instcombine does not introduce non-power-of-two integers into
+; the module
+
+target datalayout = "p:32:32:32"
+
+; This test is a counterpart to icmp_shl16 in
+; test/Transforms/InstCombine/icmp.ll, which should still pass.
+; CHECK: @icmp_shl31
+; CHECK-NOT: i31
+define i1 @icmp_shl31(i32 %x) {
+  %shl = shl i32 %x, 1
+  %cmp = icmp slt i32 %shl, 36
+  ret i1 %cmp
+}
+
+; Check that we don't introduce i4, which is a power of 2 but still not allowed.
+; CHECK: @icmp_shl4
+; CHECK-NOT: i4
+define i1 @icmp_shl4(i32 %x) {
+  %shl = shl i32 %x, 28
+  %cmp = icmp slt i32 %shl, 1073741824
+  ret i1 %cmp
+}