[clang] [clang] Use constant rounding mode for floating literals (PR #90877)

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llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-clang

Author: Serge Pavlov (spavloff)

<details>
<summary>Changes</summary>

Conversion of floating-point literal to binary representation must be made using constant rounding mode, which can be changed using pragma FENV_ROUND. For example, the literal "0.1F" should be representes by either 0.099999994 or 0.100000001 depending on the rounding direction.

---
Full diff: https://github.com/llvm/llvm-project/pull/90877.diff


4 Files Affected:

- (modified) clang/include/clang/Lex/LiteralSupport.h (+4-6) 
- (modified) clang/lib/Lex/LiteralSupport.cpp (+3-3) 
- (modified) clang/lib/Sema/SemaExpr.cpp (+2-1) 
- (modified) clang/test/AST/const-fpfeatures.c (+6) 


``````````diff
diff --git a/clang/include/clang/Lex/LiteralSupport.h b/clang/include/clang/Lex/LiteralSupport.h
index 2ed42d1c5f9aae..705021fcfa5b11 100644
--- a/clang/include/clang/Lex/LiteralSupport.h
+++ b/clang/include/clang/Lex/LiteralSupport.h
@@ -118,12 +118,10 @@ class NumericLiteralParser {
   /// bits of the result and return true.  Otherwise, return false.
   bool GetIntegerValue(llvm::APInt &Val);
 
-  /// GetFloatValue - Convert this numeric literal to a floating value, using
-  /// the specified APFloat fltSemantics (specifying float, double, etc).
-  /// The optional bool isExact (passed-by-reference) has its value
-  /// set to true if the returned APFloat can represent the number in the
-  /// literal exactly, and false otherwise.
-  llvm::APFloat::opStatus GetFloatValue(llvm::APFloat &Result);
+  /// Convert this numeric literal to a floating value, using the specified
+  /// APFloat fltSemantics (specifying float, double, etc) and rounding mode.
+  llvm::APFloat::opStatus GetFloatValue(llvm::APFloat &Result,
+                                        llvm::RoundingMode RM);
 
   /// GetFixedPointValue - Convert this numeric literal value into a
   /// scaled integer that represents this value. Returns true if an overflow
diff --git a/clang/lib/Lex/LiteralSupport.cpp b/clang/lib/Lex/LiteralSupport.cpp
index 9c0cbea5052cb2..3df0391bdda772 100644
--- a/clang/lib/Lex/LiteralSupport.cpp
+++ b/clang/lib/Lex/LiteralSupport.cpp
@@ -1520,7 +1520,8 @@ bool NumericLiteralParser::GetIntegerValue(llvm::APInt &Val) {
 }
 
 llvm::APFloat::opStatus
-NumericLiteralParser::GetFloatValue(llvm::APFloat &Result) {
+NumericLiteralParser::GetFloatValue(llvm::APFloat &Result,
+                                    llvm::RoundingMode RM) {
   using llvm::APFloat;
 
   unsigned n = std::min(SuffixBegin - ThisTokBegin, ThisTokEnd - ThisTokBegin);
@@ -1534,8 +1535,7 @@ NumericLiteralParser::GetFloatValue(llvm::APFloat &Result) {
     Str = Buffer;
   }
 
-  auto StatusOrErr =
-      Result.convertFromString(Str, APFloat::rmNearestTiesToEven);
+  auto StatusOrErr = Result.convertFromString(Str, RM);
   assert(StatusOrErr && "Invalid floating point representation");
   return !errorToBool(StatusOrErr.takeError()) ? *StatusOrErr
                                                : APFloat::opInvalidOp;
diff --git a/clang/lib/Sema/SemaExpr.cpp b/clang/lib/Sema/SemaExpr.cpp
index 0cca02c338954a..db96c8692f2516 100644
--- a/clang/lib/Sema/SemaExpr.cpp
+++ b/clang/lib/Sema/SemaExpr.cpp
@@ -3858,7 +3858,8 @@ static Expr *BuildFloatingLiteral(Sema &S, NumericLiteralParser &Literal,
   using llvm::APFloat;
   APFloat Val(Format);
 
-  APFloat::opStatus result = Literal.GetFloatValue(Val);
+  APFloat::opStatus result =
+      Literal.GetFloatValue(Val, S.CurFPFeatures.getRoundingMode());
 
   // Overflow is always an error, but underflow is only an error if
   // we underflowed to zero (APFloat reports denormals as underflow).
diff --git a/clang/test/AST/const-fpfeatures.c b/clang/test/AST/const-fpfeatures.c
index 6600ea27405d9c..247245c776fed1 100644
--- a/clang/test/AST/const-fpfeatures.c
+++ b/clang/test/AST/const-fpfeatures.c
@@ -19,6 +19,9 @@ float FI1u = 0xFFFFFFFFU;
 float _Complex C1u = C0;
 // CHECK: @C1u = {{.*}} { float, float } { float 0x3FF0000020000000, float 0x3FF0000020000000 }
 
+float FLu = 0.1F;
+// CHECK: @FLu = {{.*}} float 0x3FB9999980000000
+
 
 #pragma STDC FENV_ROUND FE_DOWNWARD
 
@@ -35,3 +38,6 @@ float FI1d = 0xFFFFFFFFU;
 
 float _Complex C1d = C0;
 // CHECK: @C1d = {{.*}} { float, float } { float 1.000000e+00, float 1.000000e+00 }
+
+float FLd = 0.1F;
+// CHECK: @FLd = {{.*}} float 0x3FB99999A0000000

``````````

</details>


https://github.com/llvm/llvm-project/pull/90877