[ValueTracking] Support funnel shifts in computeKnownBits()

If the shift amount is known, we can determine the known bits of the
output based on the known bits of two inputs.

This is essentially the same functionality as implemented in D54869,
but for ValueTracking rather than InstCombine SimplifyDemandedBits.

Differential Revision: https://reviews.llvm.org/D55140

llvm-svn: 348091

1 files changed, 48 insertions, 0 deletions

diff --git a/llvm/unittests/Analysis/ValueTrackingTest.cpp b/llvm/unittests/Analysis/ValueTrackingTest.cpp
index eeefb605867..5b36d63b390 100644
--- a/llvm/unittests/Analysis/ValueTrackingTest.cpp
+++ b/llvm/unittests/Analysis/ValueTrackingTest.cpp
@@ -568,3 +568,51 @@ TEST_F(ComputeKnownBitsTest, ComputeKnownMulBits) {
       "}\n");
   expectKnownBits(/*zero*/ 95u, /*one*/ 32u);
 }
+
+TEST_F(ComputeKnownBitsTest, ComputeKnownFshl) {
+  // fshl(....1111....0000, 00..1111........, 6)
+  // = 11....000000..11
+  parseAssembly(
+      "define i16 @test(i16 %a, i16 %b) {\n"
+      "  %aa = shl i16 %a, 4\n"
+      "  %bb = lshr i16 %b, 2\n"
+      "  %aaa = or i16 %aa, 3840\n"
+      "  %bbb = or i16 %bb, 3840\n"
+      "  %A = call i16 @llvm.fshl.i16(i16 %aaa, i16 %bbb, i16 6)\n"
+      "  ret i16 %A\n"
+      "}\n"
+      "declare i16 @llvm.fshl.i16(i16, i16, i16)\n");
+  expectKnownBits(/*zero*/ 1008u, /*one*/ 49155u);
+}
+
+TEST_F(ComputeKnownBitsTest, ComputeKnownFshr) {
+  // fshr(....1111....0000, 00..1111........, 26)
+  // = 11....000000..11
+  parseAssembly(
+      "define i16 @test(i16 %a, i16 %b) {\n"
+      "  %aa = shl i16 %a, 4\n"
+      "  %bb = lshr i16 %b, 2\n"
+      "  %aaa = or i16 %aa, 3840\n"
+      "  %bbb = or i16 %bb, 3840\n"
+      "  %A = call i16 @llvm.fshr.i16(i16 %aaa, i16 %bbb, i16 26)\n"
+      "  ret i16 %A\n"
+      "}\n"
+      "declare i16 @llvm.fshr.i16(i16, i16, i16)\n");
+  expectKnownBits(/*zero*/ 1008u, /*one*/ 49155u);
+}
+
+TEST_F(ComputeKnownBitsTest, ComputeKnownFshlZero) {
+  // fshl(....1111....0000, 00..1111........, 0)
+  // = ....1111....0000
+  parseAssembly(
+      "define i16 @test(i16 %a, i16 %b) {\n"
+      "  %aa = shl i16 %a, 4\n"
+      "  %bb = lshr i16 %b, 2\n"
+      "  %aaa = or i16 %aa, 3840\n"
+      "  %bbb = or i16 %bb, 3840\n"
+      "  %A = call i16 @llvm.fshl.i16(i16 %aaa, i16 %bbb, i16 0)\n"
+      "  ret i16 %A\n"
+      "}\n"
+      "declare i16 @llvm.fshl.i16(i16, i16, i16)\n");
+  expectKnownBits(/*zero*/ 15u, /*one*/ 3840u);
+}