CC'ing Denis
Am 26.05.26 um 15:15 schrieb Georg-Johann Lay:
This patch adds support for [[len=<words>]] in (the comments of) inline
asm constructs. It serves several purposes:
- Cases where the expanded asm is longer than determined from the number
of physical and logical line breaks. Such cases can lead to errors
when a jump that uses a too optimistic jump offset is crossing an asm.
- Better code generation for jumps that are crossing an asm.
The default length of an asm is (1 + NL) * 2 words, where NL
denotes the sum of physical and logical line breaks. However,
almost all AVR instructions occupy only one 16-bit word.
The feature is implemented in ADJUST_INSN_LENGTH. The length of
an asm is the sum over all [[len=<words>]] notes, except when an
unrecognized construct is found or an error occurred. In the latter
case, the default insn length is used. These <words> are supported:
<words> = [0-9]+
Specifies a non-negative decimal integer.
<words> = %[0-9]+
<words> = %[<name>] # Already resolved to %[0-9]+ by the middle-end.
Refers to the respective asm operand, which must be CONST_INT.
<words> = lds
<words> = sts
Specifies the length of a LDS or STS instruction, i.e.
1 word if AVR_TINY, and 2 words otherwise.
<words> = %~
<words> = %~call
<words> = %~jmp
Specifies the length of a %~call resp. %~jmp instruction, i.e.
2 words if AVR_HAVE_JMP_CALL, and 1 word otherwise.
In order to observe the assigned lengths, see -fdump-rtl-shorten or the
";; ADDR = ..." insn addresses in the asm output with -mlog=insn_addresses.
The benefits of using magic comments are:
- The feature is backwards compatible, and the target code can use
the same asm syntax since only asm comments have to be adjusted.
No #ifdef feature test macros are needed. The only case where the
feature is not fully backwards compatible is when asm templates
already contain invalid "[[len=" notes for some reason. In that
case, -mno-asm-len-notes restores the old behavior.
- Since the asm size is the sum over all notes, the final size can
be stitched together from multiple annotations / parts of an asm
template, and there is no need to support operations like plus.
There are still many asm constructs in AVR-LibC for example, like
in avr/pgmspace.h which is still used in C++ because avr-g++
doesn not (yet) support named address-spaces like __flash.
The patch passes without new regressions.
Before going all in and adding test cases etc. my question is if
this feature is okay in principle, and if magic comments other
than [[len=...]] would be preferred for some reason.
The only thing that doesn't work as expected (though expected
by the LTO designers), is using warning_at in lto1. This means
that in order to get the warnings in LTO mode, one has to compile
with -ffat-lto-objects so cc1[plus] can do the diagnosing.
The patch is for trunk.
Johann
--
AVR: Support [[len=<words]] notes in inline asm to specify its size.
This patch adds support for [[len=<words>]] in (the comments of) inline
asm constructs. It serves several purposes:
- Cases where the expanded asm is longer than determined from the number
of physical and logical line breaks. Such cases can lead to errors
when a jump that uses a too optimistic jump offset is crossing an asm.
- Better code generation for jumps that are crossing an asm. The default
length of an asm is (1 + NL) * 2 words, where NL denotes the sum of
physical and logical line breaks. However, almost all AVR instructions
occupy only one 16-bit word.
The feature is implemented in ADJUST_INSN_LENGTH. The length of
an asm is the sum over all [[len=<words>]] notes, except when an
unrecognized construct is found or an error occurred. In the latter
case, the default insn length is used. These <words> are supported:
<words> = [0-9]+
Specifies a non-negative decimal integer.
<words> = %[0-9]+
<words> = %[<name>] # Already resolved to %[0-9]+ by the middle-end.
Refers to the respective asm operand, which must be CONST_INT.
<words> = lds
<words> = sts
Specifies the length of a LDS or STS instruction, i.e.
1 word if AVR_TINY, and 2 words otherwise.
<words> = %~
<words> = %~call
<words> = %~jmp
Specifies the length of a %~call resp. %~jmp instruction, i.e.
2 words if AVR_HAVE_JMP_CALL, and 1 word otherwise.
In order to observe the assigned lengths, see -fdump-rtl-shorten or the
";; ADDR = ..." insn addresses in the asm output with -mlog=insn_addresses.
The benefits of using magic comments are:
- The feature is backwards compatible, and the target code can use
the same asm syntax since only asm comments have to be adjusted.
No #ifdef feature test macros are needed. The only case where the
feature is not fully backwards compatible is when asm templates
already contain invalid "[[len=" notes for some reason. In that
case, -mno-asm-len-notes restores the old behavior.
- Since the asm size is the sum over all notes, the final size can
be stitched together from multiple annotations / parts of an asm
template, and there is no need to support operations like plus.
gcc/
* config/avr/avr.cc (avr_read_number, avr_length_of_asm)
(avr_maybe_length_of_asm): New static functions.
(avr_adjust_insn_length): Call avr_maybe_length_of_asm on
unrecognized insns.
* config/avr/avr.opt (-masm-len-notes, -Wasm-len-notes): New
options.
* doc/invoke.texi (AVR Options): Add -masm-len-notes,
-Wasm-len-notes.
* doc/extend.texi (Size of an asm): Add @subsubheading
"Specifying the size of an asm on AVR".
libgcc/config/avr/libf7/
* libf7.h: Add "[len=...]]" notes to all non-empty inline asm's.
* libf7.c: Dito.
