https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114673
Bug ID: 114673
Summary: RISC-V: "L" constraint cannot be used for lui in
inline asm
Product: gcc
Version: 14.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: target
Assignee: unassigned at gcc dot gnu.org
Reporter: cmuellner at gcc dot gnu.org
Target Milestone: ---
The RISC-V-specific "L" constraint is neither documented nor tested.
In constraints.md it is defined as "A U-type 20-bit signed immediate.".
It tests if the value is a constant int that satisfies LUI_OPERAND(),
i.e. a value with the lowest 12 bits zero.
One obvious use-case is to use "L" for "lui" in inline asm.
However, it does not work as expected:
long getB()
{
//lui a0,0x1800
return 3<<23; //0x01800000
}
long getB_asm_i()
{
long reg;
//lui a0,0x1800
asm("lui %0, %1" : "=r"(reg) : "i"((3<<23) >> 12));
return reg;
}
long getB_asm_L()
{
long reg;
//Assembler error: lui expression not in range 0..1048575
asm("lui %0, %1" : "=r"(reg) : "L"(3ul<<23));
return reg;
}
long getB_asm_Lshift()
{
long reg;
//Compiler error: impossible constraint in 'asm'
asm("lui %0, %1" : "=r"(reg) : "L"((3<<23) >> 12));
return reg;
}
The "L" constraint was introduced as part of the initial RISC-V port.
I could not find any tests/documentation, so I am unsure if it can be fixed
or if a new constraint should be introduced.
My preferred fix would be to shift the provided constant right by 12
if it satisfies LUI_OPERAND(), so that getB_asm_L() would work.
