Extend known bits to understand @llvm.bswap

This is a cleaned up patch from the one written by John Regehr based on the findings of the Souper superoptimizer.

When writing tests, I was surprised to find that instsimplify apparently doesn't know how to collapse bit test sequences based purely on known bits. This required me to split my tests across both instsimplify and instcombine.

Differential Revision: http://reviews.llvm.org/D13250

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@249453 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/lib/Analysis/ValueTracking.cpp b/lib/Analysis/ValueTracking.cpp
index f0b04b7133d05b19d2994ee4973d50093e718fc5..d3723037ddaf4fd0204db4d345218319800c392e 100644 (file)
--- a/lib/Analysis/ValueTracking.cpp
+++ b/lib/Analysis/ValueTracking.cpp
@@ -1358,6 +1358,12 @@ static void computeKnownBitsFromOperator(Operator *I, APInt &KnownZero,
     if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(I)) {
       switch (II->getIntrinsicID()) {
       default: break;
+      case Intrinsic::bswap:
+        computeKnownBits(I->getOperand(0), KnownZero2, KnownOne2, DL,
+                         Depth + 1, Q);
+        KnownZero |= KnownZero2.byteSwap();
+        KnownOne |= KnownOne2.byteSwap();
+        break;
       case Intrinsic::ctlz:
       case Intrinsic::cttz: {
         unsigned LowBits = Log2_32(BitWidth)+1;

diff --git a/test/Transforms/InstCombine/bswap-known-bits.ll b/test/Transforms/InstCombine/bswap-known-bits.ll
new file mode 100644 (file)
index 0000000..1f3285a
--- /dev/null
+++ b/test/Transforms/InstCombine/bswap-known-bits.ll
@@ -0,0 +1,47 @@
+; RUN: opt < %s -S -instcombine | FileCheck %s
+; Note: This is testing functionality in computeKnownBits.  I'd have rather
+; used instsimplify, but the bit test folding is apparently only in instcombine.
+
+declare i16 @llvm.bswap.i16(i16)
+declare i32 @llvm.bswap.i32(i32)
+
+define i1 @test1(i16 %arg) {
+; CHECK-LABEL: @test1
+; CHECK: ret i1 true
+  %a = or i16 %arg, 511
+  %b = call i16 @llvm.bswap.i16(i16 %a)
+  %and = and i16 %b, 256
+  %res = icmp eq i16 %and, 256
+  ret i1 %res
+}
+
+define i1 @test2(i16 %arg) {
+; CHECK-LABEL: @test2
+; CHECK: ret i1 true
+  %a = or i16 %arg, 1
+  %b = call i16 @llvm.bswap.i16(i16 %a)
+  %and = and i16 %b, 256
+  %res = icmp eq i16 %and, 256
+  ret i1 %res
+}
+
+
+define i1 @test3(i16 %arg) {
+; CHECK-LABEL: @test3
+; CHECK: ret i1 true
+  %a = or i16 %arg, 256
+  %b = call i16 @llvm.bswap.i16(i16 %a)
+  %and = and i16 %b, 1
+  %res = icmp eq i16 %and, 1
+  ret i1 %res
+}
+
+define i1 @test4(i32 %arg) {
+; CHECK-LABEL: @test4
+; CHECK: ret i1 true
+  %a = or i32 %arg, 2147483647  ; i32_MAX
+  %b = call i32 @llvm.bswap.i32(i32 %a)
+  %and = and i32 %b, 127
+  %res = icmp eq i32 %and, 127
+  ret i1 %res
+}

diff --git a/test/Transforms/InstSimplify/bswap.ll b/test/Transforms/InstSimplify/bswap.ll
new file mode 100644 (file)
index 0000000..7bc3af9
--- /dev/null
+++ b/test/Transforms/InstSimplify/bswap.ll
@@ -0,0 +1,41 @@
+; RUN: opt < %s -S -instsimplify | FileCheck %s
+
+declare i16 @llvm.bswap.i16(i16)
+
+define i1 @test1(i16 %arg) {
+; CHECK-LABEL: @test1
+; CHECK: ret i1 false
+  %a = or i16 %arg, 1
+  %b = call i16 @llvm.bswap.i16(i16 %a)
+  %res = icmp eq i16 %b, 0
+  ret i1 %res
+}
+
+define i1 @test2(i16 %arg) {
+; CHECK-LABEL: @test2
+; CHECK: ret i1 false
+  %a = or i16 %arg, 1024
+  %b = call i16 @llvm.bswap.i16(i16 %a)
+  %res = icmp eq i16 %b, 0
+  ret i1 %res
+}
+
+define i1 @test3(i16 %arg) {
+; CHECK-LABEL: @test3
+; CHECK: ret i1 false
+  %a = and i16 %arg, 1
+  %b = call i16 @llvm.bswap.i16(i16 %a)
+  %and = and i16 %b, 1
+  %res = icmp eq i16 %and, 1
+  ret i1 %res
+}
+
+define i1 @test4(i16 %arg) {
+; CHECK-LABEL: @test4
+; CHECK: ret i1 false
+  %a = and i16 %arg, 511
+  %b = call i16 @llvm.bswap.i16(i16 %a)
+  %and = and i16 %b, 256
+  %res = icmp eq i16 %and, 1
+  ret i1 %res
+}