[llvm] [APInt] Remove accumulator initialization from tcMultiply and tcFullMultiply. NFCI (PR #88202)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-support

Author: Craig Topper (topperc)

<details>
<summary>Changes</summary>

The tcMultiplyPart routine has a flag that says whether to add to the accumulator or overwrite it. By using the overwrite mode on the first iteration we don't need to initialize the accumulator to zero.

Note, the initialization in tcFullMultiply was only initializing the first rhsParts of dst. tcMultiplyPart always overwrites the rhsParts+1 part that just contains the last carry. The first write to each part of dst past rhsParts is a carry write so that's how the upper part of dst is initialized.

---
Full diff: https://github.com/llvm/llvm-project/pull/88202.diff


1 Files Affected:

- (modified) llvm/lib/Support/APInt.cpp (+10-7) 


``````````diff
diff --git a/llvm/lib/Support/APInt.cpp b/llvm/lib/Support/APInt.cpp
index 8825025ec32134..599c08a3c2b938 100644
--- a/llvm/lib/Support/APInt.cpp
+++ b/llvm/lib/Support/APInt.cpp
@@ -2585,11 +2585,13 @@ int APInt::tcMultiply(WordType *dst, const WordType *lhs,
   assert(dst != lhs && dst != rhs);
 
   int overflow = 0;
-  tcSet(dst, 0, parts);
 
-  for (unsigned i = 0; i < parts; i++)
+  for (unsigned i = 0; i < parts; i++) {
+    // Don't accumulate on the first iteration so we don't need to initalize
+    // dst to 0.
     overflow |= tcMultiplyPart(&dst[i], lhs, rhs[i], 0, parts,
-                               parts - i, true);
+                               parts - i, i != 0);
+  }
 
   return overflow;
 }
@@ -2605,10 +2607,11 @@ void APInt::tcFullMultiply(WordType *dst, const WordType *lhs,
 
   assert(dst != lhs && dst != rhs);
 
-  tcSet(dst, 0, rhsParts);
-
-  for (unsigned i = 0; i < lhsParts; i++)
-    tcMultiplyPart(&dst[i], rhs, lhs[i], 0, rhsParts, rhsParts + 1, true);
+  for (unsigned i = 0; i < lhsParts; i++) {
+    // Don't accumulate on the first iteration so we don't need to initalize
+    // dst to 0.
+    tcMultiplyPart(&dst[i], rhs, lhs[i], 0, rhsParts, rhsParts + 1, i != 0);
+  }
 }
 
 // If RHS is zero LHS and REMAINDER are left unchanged, return one.

``````````

</details>


https://github.com/llvm/llvm-project/pull/88202