[ARM] Improve comment on the 'J' inline asm modifier. #160712
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An inline asm constraint "Jr", in AArch32, means that if the input
value is a compile-time constant in the range -4095 to +4095, then it
can be inserted into the assembly language as an immediate operand,
and otherwise it will be placed in a register.
The comment in the Arm backend said "It is not clear what this
constraint is intended for". I believe the answer is that that range
of immediate values are the ones you can use in a LDR or STR
instruction. So it's suitable for cases like this:
asm("str %0,[%1,%2]" : : "r"(data), "r"(base), "Jr"(offset) : "memory");
in the same way that the "Ir" constraint is suitable for the immediate
in a data-processing instruction such as ADD or EOR.
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@llvm/pr-subscribers-backend-arm Author: Simon Tatham (statham-arm) ChangesAn inline asm constraint "Jr", in AArch32, means that if the input value is a compile-time constant in the range -4095 to +4095, then it can be inserted into the assembly language as an immediate operand, and otherwise it will be placed in a register. The comment in the Arm backend said "It is not clear what this constraint is intended for". I believe the answer is that that range of immediate values are the ones you can use in a LDR or STR instruction. So it's suitable for cases like this: asm("str %0,[%1,%2]" : : "r"(data), "r"(base), "Jr"(offset) : "memory"); in the same way that the "Ir" constraint is suitable for the immediate in a data-processing instruction such as ADD or EOR. Full diff: https://github.com/llvm/llvm-project/pull/160712.diff 1 Files Affected:
diff --git a/llvm/lib/Target/ARM/ARMISelLowering.cpp b/llvm/lib/Target/ARM/ARMISelLowering.cpp
index 9a247bb5a83d9..c283b0c77872e 100644
--- a/llvm/lib/Target/ARM/ARMISelLowering.cpp
+++ b/llvm/lib/Target/ARM/ARMISelLowering.cpp
@@ -20428,9 +20428,9 @@ void ARMTargetLowering::LowerAsmOperandForConstraint(SDValue Op,
if (CVal >= -255 && CVal <= -1)
break;
} else {
- // This must be a constant between -4095 and 4095. It is not clear
- // what this constraint is intended for. Implemented for
- // compatibility with GCC.
+ // This must be a constant between -4095 and 4095. This is suitable
+ // for use as the immediate offset field in LDR and STR instructions
+ // such as LDR r0,[r1,#offset].
if (CVal >= -4095 && CVal <= 4095)
break;
}
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LGTM
The only reference I can find is the Arm inline assembly cookbook which has a table near to For ARM processors, GCC 4 provides the following constraints. in:
http://www.ethernut.de/en/documents/arm-inline-asm.html
It does mention a similar example to yours.
An inline asm constraint "Jr", in AArch32, means that if the input value
is a compile-time constant in the range -4095 to +4095, then it can be
inserted into the assembly language as an immediate operand, and
otherwise it will be placed in a register.
The comment in the Arm backend said "It is not clear what this
constraint is intended for". I believe the answer is that that range of
immediate values are the ones you can use in a LDR or STR instruction.
So it's suitable for cases like this:
asm("str %0,[%1,%2]" : : "r"(data), "r"(base), "Jr"(offset) : "memory");
in the same way that the "Ir" constraint is suitable for the immediate
in a data-processing instruction such as ADD or EOR.
An inline asm constraint "Jr", in AArch32, means that if the input value is a compile-time constant in the range -4095 to +4095, then it can be inserted into the assembly language as an immediate operand, and otherwise it will be placed in a register.
The comment in the Arm backend said "It is not clear what this constraint is intended for". I believe the answer is that that range of immediate values are the ones you can use in a LDR or STR instruction. So it's suitable for cases like this:
asm("str %0,[%1,%2]" : : "r"(data), "r"(base), "Jr"(offset) : "memory");
in the same way that the "Ir" constraint is suitable for the immediate in a data-processing instruction such as ADD or EOR.