Handle large imm values in ARMEmitter.EmitLDR (dotnet#98433)

hwoodiwiss · Feb 14, 2024 · b2e93f8 · b2e93f8
1 parent d3ebc97
commit b2e93f8
Showing 1 changed file with 32 additions and 0 deletions.
diff --git a/src/coreclr/tools/Common/Compiler/DependencyAnalysis/Target_ARM/ARMEmitter.cs b/src/coreclr/tools/Common/Compiler/DependencyAnalysis/Target_ARM/ARMEmitter.cs
@@ -121,9 +121,41 @@ public void EmitLDR(Register destination, Register source)
         // ldr.w reg, [reg, #offset]
         // reg range: [0..PC]
         // offset range: [-255..4095]
+        //
+        // for offset >= 4096 we do an expansion into:
+        // add.w destination, source, #const
+        // ldr.w destination, [destination, #offset]
         public void EmitLDR(Register destination, Register source, int offset)
         {
             Debug.Assert(IsValidReg(destination) && IsValidReg(source));
+
+            if (offset >= 0x1000)
+            {
+                uint constVal = (uint)offset & ~0xfffu;
+                uint mask32 = 0xff;
+                uint imm8 = 0;
+                int encode = 31; // 11111
+
+                do
+                {
+                    mask32 <<= 1;
+                    if ((constVal & ~mask32) == 0)
+                    {
+                        imm8 = (constVal & mask32) >> (32 - encode);
+                        break;
+                    }
+                    encode--;
+                } while (encode >= 8);
+
+                Debug.Assert(encode >= 8);
+                Debug.Assert((imm8 & 0x80) > 0);
+                Builder.EmitShort((short)(0xF100 + (byte)source + (((byte)encode & 0x10) << 6)));
+                Builder.EmitShort((short)((((byte)encode & 0xE) << 11) + ((byte)destination << 8) + (((byte)encode & 1) << 7) + (imm8 & 0x7f)));
+
+                offset = (int)(offset & 0xfffu);
+                source = destination;
+            }
+
             Debug.Assert(offset >= -255 && offset <= 4095);
             if (offset >= 0)
             {