[llvm] [APInt] remove getClearedMemory and improve slow initialization (PR #106945)

[Nikita Popov via llvm-commits]

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@@ -74,11 +66,14 @@ inline static unsigned getDigit(char cdigit, uint8_t radix) {
 
 
 void APInt::initSlowCase(uint64_t val, bool isSigned) {
-  U.pVal = getClearedMemory(getNumWords());
-  U.pVal[0] = val;
-  if (isSigned && int64_t(val) < 0)
-    for (unsigned i = 1; i < getNumWords(); ++i)
-      U.pVal[i] = WORDTYPE_MAX;
+  if (isSigned && int64_t(val) < 0) {
+    U.pVal = new WordType[getNumWords()];
+    U.pVal[0] = val;
+    memset(&U.pVal[1], 0xFF, sizeof(WordType) * (getNumWords() - 1));
+  } else {
+    U.pVal = new WordType[getNumWords()]();
+    U.pVal[0] = val;
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nikic wrote:

This doesn't look correct to me. Not that clearUnusedBits() only clears bits in the high order word, so you will leave everything between the high order word and the low order word uninitialized.

https://github.com/llvm/llvm-project/pull/106945