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This allows you to explicitly specify the section to emit the data
directive for, allowing for sections other than .data: for example, .bss
or .init_array.
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GNU ld uses the presence of these notes to determine the flags of
the final GNU_STACK program header. If they are present in every
object, then the resulting executable's GNU_STACK uses flags RW
instead of RWE.
Reported by Érico Nogueira Rolim.
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The immediate in the add instruction is only 12 bits. If the offset
does not fit, we must move it into a register first.
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Otherwise, if a constant is stored as a float and retrieved as an
int, the padding bits are uninitialized. This can result in the
generation of invalid assembly:
Error: suffix or operands invalid for `cvtsi2ss'
Reported by Hiltjo Posthuma.
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Thanks to Jakob for pointing this out.
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Otherwise, we may end up using an integer and floating class for the
same register, triggering an assertion failure:
qbe: rega.c:215: pmrec: Assertion `KBASE(pm[i].cls) == KBASE(*k)' failed.
Test case:
type :T = { s }
export
function $d(:T %.1, s %.2) {
@start
call $c(s %.2)
ret
}
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According to the ARMv8 overview document
However if SP is used as the base register then the value of the stack
pointer prior to adding any offset must be quadword (16 byte) aligned,
or else a stack alignment exception will be generated.
This manifests as a bus error on my system.
To resolve this, just save registers two at a time with stp.
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This now only limits the number of arguments when parsing the input SSA,
which is usually a small fixed size (depending on the frontend).
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C99 6.5.2.5p6:
> If the compound literal occurs outside the body of a function,
> the object has static storage duration; otherwise, it has automatic
> storage duration associated with the enclosing block.
So, we can't use the address of a compound literal here. Instead,
just set p to NULL, and make the loop conditional on p being non-NULL.
Remarks from Quentin:
I made a cosmetic change to Michael's
original patch and merely pushed the
literal at toplevel.
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The mandel example uses SDL2 for graphics
output. When GCC is used to assemble the
resulting *.s file it shows linker's
errors about undefined symbols from the
library.
This behavior can be fixed by moving
the flags passed to the compiler after
the source file name.
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In this case, the immediate is too large to use directly in the add/sub
instructions, so move it into a temporary register first.
Also, for clarity, rearrange the if-conditions so that they match the
constraints of the instructions that immediately follow.
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The ldrb and ldrh instructions require a 32-bit register name for the
destination and will clear the upper 32-bits of that register.
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The code used to see add 0, 10 as
a copy of 0.
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The same functionality can be implemented
naturally in the cfg simplification pass.
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Previously, each ret would lead to an
epilog. This caused bloat for large
functions with multiple return points.
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.align N can either mean align to
the next multiple of N or align to
the next multiple of 1<<N.
Credit goes to Jorge Acereda Maciá
for reporting this issue.
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SCCP is currently the one and only
pass which seriously affects control
flow; so we must compute loop costs
afterwards.
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When lowering pointer arithmetic, it is
natural for a C frontend to generate
those instructions.
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The heuristic was bogus for at least
two reasons (see below), and, looking
at some generated code, it looks like
some other issues are more pressing.
1. A stack slot of 4 bytes could be
used for a temporary of 8 bytes.
2. Should 2 arguments of an operation
end up spilled, the same slot
could be allocated to both!
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The value argument of store instructions was
handled incorrectly.
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This fixes similar bugs than the ones fixed
in the previous commit.
In the folding code the invariant is that
when a result is 32 bits wide, the low 32
bits of 'x' are correct. The high bits
can be anything.
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This prevents an FE_INVALID exception when comparing with NaN.
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Thanks to Jorge Acereda Maciá for catching
this.
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This was generated by csmith and then compiled
to qbe il by Michael Forney's C compiler.
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Slots are stored as `int` in Fn, so use the same type in Tmp.
Rearrange the fields in Tmp slightly so that sizeof(Tmp) stays the same
(at least on 64-bit systems).
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I had forgotten that %rip can only be
used as base when there is no index.
I also added a test which stresses
addressing selection with and without
constants.
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We now emit correct code when the user
refers to a specific constant address.
I also made some comments clearer in
the instruction selection pass and got
rid of some apparently useless code.
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In this case, the potential truncations
flagged by gcc are only affecting debug
information.
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Michael Forney needs this to run his
compiler on interesting programs.
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There is no flavor of mov which can set 8 bytes
of memory to a constant not representable as an
int32. The solution is simply to emit two movs
of 4 bytes each.
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Previously, we would skip ssa construction when
a temporary has a single definition. This is
only part of the ssa invariant: we must also
check that all uses are dominated by the single
definition. The new code does this.
In fact, qbe does not store all the dominators
for a block, so instead of walking the idom
linked list we use a rough heuristic and declare
conservatively that B0 dominates B1 when one of
the two conditions is true:
a. B0 is the start block
b. B0 is B1
Some measurements on a big file from Michael
Forney show that the code is still as fast as
before this patch.
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