[flang-commits] [flang] [flang] Fix spurious overflow warning folding exponentiation by integ… (PR #88188)

[Peter Klausler via flang-commits]

https://github.com/klausler created https://github.com/llvm/llvm-project/pull/88188

…er powers

The code that folds exponentiation by an integer power can report a spurious overflow warning because it calculates one last unnecessary square of the base value.  10.**(+/-32) exposes the problem -- the value of 10.**64 is calculated but not needed.  Rearrange the implementation to only calculate squares that are necessary.

Fixes https://github.com/llvm/llvm-project/issues/88151.

>From fb573c4b07c6b4cc51cdd9e8260ddbf9e32f920a Mon Sep 17 00:00:00 2001
From: Peter Klausler <pklausler at nvidia.com>
Date: Tue, 9 Apr 2024 13:29:07 -0700
Subject: [PATCH] [flang] Fix spurious overflow warning folding exponentiation
 by integer powers

The code that folds exponentiation by an integer power can report
a spurious overflow warning because it calculates one last unnecessary
square of the base value.  10.**(+/-32) exposes the problem --
the value of 10.**64 is calculated but not needed.  Rearrange the
implementation to only calculate squares that are necessary.

Fixes https://github.com/llvm/llvm-project/issues/88151.
---
 flang/lib/Evaluate/int-power.h | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/flang/lib/Evaluate/int-power.h b/flang/lib/Evaluate/int-power.h
index 0d6a133ae73c51..2ee012ceb77a38 100644
--- a/flang/lib/Evaluate/int-power.h
+++ b/flang/lib/Evaluate/int-power.h
@@ -33,6 +33,10 @@ ValueWithRealFlags<REAL> TimesIntPowerOf(const REAL &factor, const REAL &base,
     REAL squares{base};
     int nbits{INT::bits - absPower.LEADZ()};
     for (int j{0}; j < nbits; ++j) {
+      if (j > 0) { // avoid spurious overflow on last iteration
+        squares =
+            squares.Multiply(squares, rounding).AccumulateFlags(result.flags);
+      }
       if (absPower.BTEST(j)) {
         if (negativePower) {
           result.value = result.value.Divide(squares, rounding)
@@ -42,8 +46,6 @@ ValueWithRealFlags<REAL> TimesIntPowerOf(const REAL &factor, const REAL &base,
                              .AccumulateFlags(result.flags);
         }
       }
-      squares =
-          squares.Multiply(squares, rounding).AccumulateFlags(result.flags);
     }
   }
   return result;