ADT: Fix that APSInt's string constructor claims it requires 5 bits to store a zero

[teemperor]

Summary

When constructing an APSInt from a string, the constructor doesn't correctly
truncate the bit width of the result if the passed in string was "0" (or any alternative
way to express 0 like "-0" or "000"). Instead of 1 (which is the smallest allowed bit
width) it returns an APSInt with a bit width of 5.

The reason is that the constructor checks that it never truncates the result to the
invalid bit width of 0, so when it calculates that storing a "0" doesn't require any
bits it just keeps the original overestimated bit width (which happens to be 5).

This patch just sets the bit width of the result to 1 if the required bit width is 0.

[teemperor]

• Rename the test.

[dexonsmith]

LGTM.

llvm/lib/Support/APSInt.cpp

 APSInt::APSInt(StringRef Str) {
   assert(!Str.empty() && "Invalid string length");
 
   // (Over-)estimate the required number of bits.
   unsigned NumBits = ((Str.size() * 64) / 19) + 2;
   APInt Tmp(NumBits, Str, /*radix=*/10);
   if (Str[0] == '-') {
     unsigned MinBits = Tmp.getMinSignedBits();
-    if (MinBits > 0 && MinBits < NumBits)
-      Tmp = Tmp.trunc(MinBits);
+    if (MinBits < NumBits)
+      Tmp = Tmp.trunc(std::max<unsigned>(1, MinBits));
     *this = APSInt(Tmp, /*isUnsigned=*/false);
     return;
   }
   unsigned ActiveBits = Tmp.getActiveBits();
-  if (ActiveBits > 0 && ActiveBits < NumBits)
-    Tmp = Tmp.trunc(ActiveBits);
+  if (ActiveBits < NumBits)
+    Tmp = Tmp.trunc(std::max<unsigned>(1, ActiveBits));
   *this = APSInt(Tmp, /*isUnsigned=*/true);
 }
 
 void APSInt::Profile(FoldingSetNodeID& ID) const {
   ID.AddInteger((unsigned) (IsUnsigned ? 1 : 0));
   APInt::Profile(ID);
 }

llvm/unittests/ADT/APSIntTest.cpp

 TEST(APSIntTest, FromString) {
   EXPECT_EQ(APSInt("1").getExtValue(), 1);
   EXPECT_EQ(APSInt("-1").getExtValue(), -1);
   EXPECT_EQ(APSInt("0").getExtValue(), 0);
   EXPECT_EQ(APSInt("56789").getExtValue(), 56789);
   EXPECT_EQ(APSInt("-1234").getExtValue(), -1234);
 }
 
+TEST(APSIntTest, FromStringBitWidth) {
+  EXPECT_EQ(APSInt("0").getBitWidth(), 1U);
+  EXPECT_EQ(APSInt("000").getBitWidth(), 1U);
+  EXPECT_EQ(APSInt("1").getBitWidth(), 1U);
+  EXPECT_EQ(APSInt("2").getBitWidth(), 2U);
+  EXPECT_EQ(APSInt("3").getBitWidth(), 2U);
+  EXPECT_EQ(APSInt("003").getBitWidth(), 2U);
+  EXPECT_EQ(APSInt("15").getBitWidth(), 4U);
+  EXPECT_EQ(APSInt("16").getBitWidth(), 5U);
+  EXPECT_EQ(APSInt("17").getBitWidth(), 5U);
+
+  EXPECT_EQ(APSInt("-0").getBitWidth(), 1U);
+  EXPECT_EQ(APSInt("-000").getBitWidth(), 1U);
+  EXPECT_EQ(APSInt("-1").getBitWidth(), 1U);
+  EXPECT_EQ(APSInt("-2").getBitWidth(), 2U);
+  EXPECT_EQ(APSInt("-3").getBitWidth(), 3U);
+  EXPECT_EQ(APSInt("-003").getBitWidth(), 3U);
+  EXPECT_EQ(APSInt("-5").getBitWidth(), 4U);
+  EXPECT_EQ(APSInt("-15").getBitWidth(), 5U);
+  EXPECT_EQ(APSInt("-16").getBitWidth(), 5U);
+  EXPECT_EQ(APSInt("-17").getBitWidth(), 6U);
+}
+
 #if defined(GTEST_HAS_DEATH_TEST) && !defined(NDEBUG)
 
 TEST(APSIntTest, StringDeath) {
   EXPECT_DEATH((void)APSInt(""), "Invalid string length");
   EXPECT_DEATH((void)APSInt("1a"), "Invalid character in digit string");
 }
 
 #endif