[llvm] b43235a - [DebugInfo] Fix DwarfExpression::addConstantFP for float on big-endian

[Bjorn Pettersson via llvm-commits]

Author: Bjorn Pettersson
Date: 2020-08-20T11:48:05+02:00
New Revision: b43235a76c231cf15d28f073780e868b71c7bab0

URL: https://github.com/llvm/llvm-project/commit/b43235a76c231cf15d28f073780e868b71c7bab0
DIFF: https://github.com/llvm/llvm-project/commit/b43235a76c231cf15d28f073780e868b71c7bab0.diff

LOG: [DebugInfo] Fix DwarfExpression::addConstantFP for float on big-endian

The byte swapping, when dealing with 4 byte (float) FP constants
in DwarfExpression::addConstantFP, added in commit ef8992b9f0189005
was not correct. It always performed byte swapping using an
uint64_t value. When dealing with 4 byte values the 4 interesting
bytes ended up in the big end of the uint64_t, but later we emitted
the 4 bytes at the little end. So we ended up with zeroes being
emitted and faulty debug information.

This patch simplifies things a bit, IMHO. Using the APInt
representation throughout the function, instead of looking at
the internal representation using getRawBytes and without using
reinterpret_cast etc. And using API.byteSwap() should result in
correct byte swapping independent of APInt being 4 or 8 bytes.

Differential Revision: https://reviews.llvm.org/D86272

Added: 
    

Modified: 
    llvm/lib/CodeGen/AsmPrinter/DwarfExpression.cpp

Removed: 
    


################################################################################
diff  --git a/llvm/lib/CodeGen/AsmPrinter/DwarfExpression.cpp b/llvm/lib/CodeGen/AsmPrinter/DwarfExpression.cpp
index 4455035d8910..b0fa8645de24 100644
--- a/llvm/lib/CodeGen/AsmPrinter/DwarfExpression.cpp
+++ b/llvm/lib/CodeGen/AsmPrinter/DwarfExpression.cpp
@@ -231,14 +231,16 @@ void DwarfExpression::addConstantFP(const APFloat &APF, const AsmPrinter &AP) {
     emitOp(dwarf::DW_OP_implicit_value);
     emitUnsigned(NumBytes /*Size of the block in bytes*/);
 
-    const uint64_t *Value = API.getRawData();
-    const bool IsLittleEndian = AP.getDataLayout().isLittleEndian();
-    uint64_t Swapped = support::endian::byte_swap(
-        *Value, IsLittleEndian ? support::little : support::big);
-
-    const char *SwappedBytes = reinterpret_cast<const char *>(&Swapped);
-    for (int i = 0; i < NumBytes; ++i)
-      emitData1(SwappedBytes[i]);
+    // The loop below is emitting the value starting at least significant byte,
+    // so we need to perform a byte-swap to get the byte order correct in case
+    // of a big-endian target.
+    if (AP.getDataLayout().isBigEndian())
+      API = API.byteSwap();
+
+    for (int i = 0; i < NumBytes; ++i) {
+      emitData1(API.getZExtValue() & 0xFF);
+      API = API.lshr(8);
+    }
 
     return;
   }