[builtins] Divide shouldn't underflow if rounded result would be normal.

We were treating certain edge cases that are actually normal as denormal
results, and flushing them to zero; we shouldn't do that. Not sure this
is the cleanest way to implement this edge case, but I wanted to avoid
adding any code on the common path.

Differential Revision: https://reviews.llvm.org/D59070



git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@356529 91177308-0d34-0410-b5e6-96231b3b80d8

3 files changed, 36 insertions, 0 deletions

diff --git a/lib/builtins/divdf3.c b/lib/builtins/divdf3.c
index 3993507a0..9132f8381 100644
--- a/lib/builtins/divdf3.c
+++ b/lib/builtins/divdf3.c
@@ -162,6 +162,18 @@ __divdf3(fp_t a, fp_t b) {
     }
 
     else if (writtenExponent < 1) {
+        if (writtenExponent == 0) {
+            // Check whether the rounded result is normal.
+            const bool round = (residual << 1) > bSignificand;
+            // Clear the implicit bit.
+            rep_t absResult = quotient & significandMask;
+            // Round.
+            absResult += round;
+            if (absResult & ~significandMask) {
+                // The rounded result is normal; return it.
+                return fromRep(absResult | quotientSign);
+            }
+        }
         // Flush denormals to zero.  In the future, it would be nice to add
         // code to round them correctly.
         return fromRep(quotientSign);
diff --git a/lib/builtins/divsf3.c b/lib/builtins/divsf3.c
index 97cbcc196..944524749 100644
--- a/lib/builtins/divsf3.c
+++ b/lib/builtins/divsf3.c
@@ -147,6 +147,18 @@ __divsf3(fp_t a, fp_t b) {
     }
 
     else if (writtenExponent < 1) {
+        if (writtenExponent == 0) {
+            // Check whether the rounded result is normal.
+            const bool round = (residual << 1) > bSignificand;
+            // Clear the implicit bit.
+            rep_t absResult = quotient & significandMask;
+            // Round.
+            absResult += round;
+            if (absResult & ~significandMask) {
+                // The rounded result is normal; return it.
+                return fromRep(absResult | quotientSign);
+            }
+        }
         // Flush denormals to zero.  In the future, it would be nice to add
         // code to round them correctly.
         return fromRep(quotientSign);
diff --git a/lib/builtins/divtf3.c b/lib/builtins/divtf3.c
index 17273749b..ba5de99f9 100644
--- a/lib/builtins/divtf3.c
+++ b/lib/builtins/divtf3.c
@@ -181,6 +181,18 @@ COMPILER_RT_ABI fp_t __divtf3(fp_t a, fp_t b) {
         return fromRep(infRep | quotientSign);
     }
     else if (writtenExponent < 1) {
+        if (writtenExponent == 0) {
+            // Check whether the rounded result is normal.
+            const bool round = (residual << 1) > bSignificand;
+            // Clear the implicit bit.
+            rep_t absResult = quotient & significandMask;
+            // Round.
+            absResult += round;
+            if (absResult & ~significandMask) {
+                // The rounded result is normal; return it.
+                return fromRep(absResult | quotientSign);
+            }
+        }
         // Flush denormals to zero.  In the future, it would be nice to add
         // code to round them correctly.
         return fromRep(quotientSign);