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Handle large imm values in ARMEmitter.EmitLDR #98433

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Merged
jkotas merged 1 commit into from
Feb 14, 2024
Merged

Handle large imm values in ARMEmitter.EmitLDR #98433

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32 changes: 32 additions & 0 deletions src/coreclr/tools/Common/Compiler/DependencyAnalysis/Target_ARM/ARMEmitter.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -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)
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@jkotas jkotas Feb 14, 2024

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Can this be ever called with large negative offset?

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@filipnavara filipnavara Feb 14, 2024

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I checked the call sites and the only negative values seemed to be in the -PointerSize range.

{
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);
Comment on lines +134 to +148
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@filipnavara filipnavara Feb 14, 2024

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In case you are wondering, this part is shamelessly copied from the JIT:

runtime/src/coreclr/jit/emitarm.cpp

Lines 1349 to 1366 in c7c15f7

mask32 = 0x00000ff;
encode = 31; /* 11111 */
do
{
mask32 <<= 1;
temp = uval32 & ~mask32;
if (temp == 0)
{
imm8 = (uval32 & mask32) >> (32 - encode);
assert((imm8 & 0x80) != 0);
goto DONE;
}
encode--;
} while (encode >= 8);
assert(!"encodeModImmConst failed!");
return BAD_CODE;

The instruction encoding has several forms. We want to use the one where we produce an immediate value and rotate it using the barrel shifter into the right position to get the right constant.

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@jkotas jkotas Feb 14, 2024

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It does not look like that it can.


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)
{
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