[compiler-rt] 2495130 - [compiler-rt] Add missing carry to 128x128->256 wide multiply (#97257)

[via llvm-commits]

Author: beetrees
Date: 2024-09-24T17:51:13-07:00
New Revision: 2495130c1b5c255bbcf5ff84555026649c5fa697

URL: https://github.com/llvm/llvm-project/commit/2495130c1b5c255bbcf5ff84555026649c5fa697
DIFF: https://github.com/llvm/llvm-project/commit/2495130c1b5c255bbcf5ff84555026649c5fa697.diff

LOG: [compiler-rt] Add missing carry to 128x128->256 wide multiply (#97257)

Added: 
    

Modified: 
    compiler-rt/lib/builtins/fp_lib.h
    compiler-rt/test/builtins/Unit/multf3_test.c

Removed: 
    


################################################################################
diff  --git a/compiler-rt/lib/builtins/fp_lib.h b/compiler-rt/lib/builtins/fp_lib.h
index 0289cfd10db660..fae58497a8f80d 100644
--- a/compiler-rt/lib/builtins/fp_lib.h
+++ b/compiler-rt/lib/builtins/fp_lib.h
@@ -171,8 +171,11 @@ static __inline void wideMultiply(rep_t a, rep_t b, rep_t *hi, rep_t *lo) {
                          (sum2 & Word_FullMask) + ((sum3 << 32) & Word_HiMask);
 
   *lo = r0 + (r1 << 64);
+  // The addition above can overflow, in which case `*lo` will be less than
+  // `r0`. Carry any overflow into `hi`.
+  const bool carry = *lo < r0;
   *hi = (r1 >> 64) + (sum1 >> 96) + (sum2 >> 64) + (sum3 >> 32) + sum4 +
-        (sum5 << 32) + (sum6 << 64);
+        (sum5 << 32) + (sum6 << 64) + carry;
 }
 #undef Word_1
 #undef Word_2

diff  --git a/compiler-rt/test/builtins/Unit/multf3_test.c b/compiler-rt/test/builtins/Unit/multf3_test.c
index 543b55899ce82a..0e561551d35342 100644
--- a/compiler-rt/test/builtins/Unit/multf3_test.c
+++ b/compiler-rt/test/builtins/Unit/multf3_test.c
@@ -77,6 +77,12 @@ int main()
                      UINT64_C(0x0),
                      UINT64_C(0x0)))
         return 1;
+    // test carry between lo and hi in widening multiply
+    if (test__multf3(0x0.7fffffffffffffffffffffffffffp-16382L,
+                     0x1.7fffffffffffffffffffffffffffp+1L,
+                     UINT64_C(0x00017fffffffffff),
+                     UINT64_C(0xfffffffffffffffc)))
+      return 1;
 
 #else
     printf("skipped\n");