ARM inline asm secrets

Although I generally recommend against using GCC inline assembly, preferring instead pure assembly code in separate files, there are occasions where inline is the appropriate solution. Should one, at a time like this, turn to the GCC documentation for guidance, one must be prepared for a degree of disappointment. As it happens, much of the inline asm syntax is left entirely undocumented. This article attempts to fill in some of the blanks for the ARM target.

Constraints

Each operand of an inline asm block is described by a constraint string encoding the valid representations of the operand in the generated assembly. For example the “r” code denotes a general-purpose register. In addition to the standard constraints, ARM allows a number of special codes, only some of which are documented. The full list, including a brief description, is available in the constraints.md file in the GCC source tree. The following table is an extract from this file consisting of the codes which are meaningful in an inline asm block (a few are only useful in the machine description itself).

f Legacy FPA registers f0-f7.
t The VFP registers s0-s31.
v The Cirrus Maverick co-processor registers.
w The VFP registers d0-d15, or d0-d31 for VFPv3.
x The VFP registers d0-d7.
y The Intel iWMMX co-processor registers.
z The Intel iWMMX GR registers.
l In Thumb state the core registers r0-r7.
h In Thumb state the core registers r8-r15.
j A constant suitable for a MOVW instruction. (ARM/Thumb-2)
b Thumb only. The union of the low registers and the stack register.
I In ARM/Thumb-2 state a constant that can be used as an immediate value in a Data Processing instruction. In Thumb-1 state a constant in the range 0 to 255.
J In ARM/Thumb-2 state a constant in the range -4095 to 4095. In Thumb-1 state a constant in the range -255 to -1.
K In ARM/Thumb-2 state a constant that satisfies the I constraint if inverted. In Thumb-1 state a constant that satisfies the I constraint multiplied by any power of 2.
L In ARM/Thumb-2 state a constant that satisfies the I constraint if negated. In Thumb-1 state a constant in the range -7 to 7.
M In Thumb-1 state a constant that is a multiple of 4 in the range 0 to 1020.
N Thumb-1 state a constant in the range 0 to 31.
O In Thumb-1 state a constant that is a multiple of 4 in the range -508 to 508.
Pa In Thumb-1 state a constant in the range -510 to +510
Pb In Thumb-1 state a constant in the range -262 to +262
Ps In Thumb-2 state a constant in the range -255 to +255
Pt In Thumb-2 state a constant in the range -7 to +7
G In ARM/Thumb-2 state a valid FPA immediate constant.
H In ARM/Thumb-2 state a valid FPA immediate constant when negated.
Da In ARM/Thumb-2 state a const_int, const_double or const_vector that can be generated with two Data Processing insns.
Db In ARM/Thumb-2 state a const_int, const_double or const_vector that can be generated with three Data Processing insns.
Dc In ARM/Thumb-2 state a const_int, const_double or const_vector that can be generated with four Data Processing insns. This pattern is disabled if optimizing for space or when we have load-delay slots to fill.
Dn In ARM/Thumb-2 state a const_vector which can be loaded with a Neon vmov immediate instruction.
Dl In ARM/Thumb-2 state a const_vector which can be used with a Neon vorr or vbic instruction.
DL In ARM/Thumb-2 state a const_vector which can be used with a Neon vorn or vand instruction.
Dv In ARM/Thumb-2 state a const_double which can be used with a VFP fconsts instruction.
Dy In ARM/Thumb-2 state a const_double which can be used with a VFP fconstd instruction.
Ut In ARM/Thumb-2 state an address valid for loading/storing opaque structure types wider than TImode.
Uv In ARM/Thumb-2 state a valid VFP load/store address.
Uy In ARM/Thumb-2 state a valid iWMMX load/store address.
Un In ARM/Thumb-2 state a valid address for Neon doubleword vector load/store instructions.
Um In ARM/Thumb-2 state a valid address for Neon element and structure load/store instructions.
Us In ARM/Thumb-2 state a valid address for non-offset loads/stores of quad-word values in four ARM registers.
Uq In ARM state an address valid in ldrsb instructions.
Q In ARM/Thumb-2 state an address that is a single base register.

Operand codes

Within the text of an inline asm block, operands are referenced as %0, %1 etc. Register operands are printed as rN, memory operands as [rN, #offset], and so forth. In some situations, for example with operands occupying multiple registers, more detailed control of the output may be required, and once again, an undocumented feature comes to our rescue.

Special code letters inserted between the % and the operand number alter the output from the default for each type of operand. The table below lists the more useful ones.

c An integer or symbol address without a preceding # sign
B Bitwise inverse of integer or symbol without a preceding #
L The low 16 bits of an immediate constant
m The base register of a memory operand
M A register range suitable for LDM/STM
H The highest-numbered register of a pair
Q The least significant register of a pair
R The most significant register of a pair
P A double-precision VFP register
p The high single-precision register of a VFP double-precision register
q A NEON quad register
e The low doubleword register of a NEON quad register
f The high doubleword register of a NEON quad register
h A range of VFP/NEON registers suitable for VLD1/VST1
A A memory operand for a VLD1/VST1 instruction
y S register as indexed D register, e.g. s5 becomes d2[1]
Bookmark the permalink.

24 Responses to ARM inline asm secrets

  1. Pingback: Inline-Assembly – 문c 블로그

  2. Pingback: Kombinace assembleru a programovacího jazyka C na procesorech ARM | MojeFedora.cz

  3. Pingback: Kombinace assembleru a programovacího jazyka C na procesorech ARM (dokončení) | MojeFedora.cz

Leave a Reply to Mans Cancel reply

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.