Hi Christophe,
On Fri, Jan 15, 2021, at 4:30 AM, Christophe Lyon wrote:
> On Fri, 15 Jan 2021 at 12:39, Daniel Engel <[email protected]> wrote:
> >
> > Hi Christophe,
> >
> > On Mon, Jan 11, 2021, at 8:39 AM, Christophe Lyon wrote:
> > > On Mon, 11 Jan 2021 at 17:18, Daniel Engel <[email protected]> wrote:
> > > >
> > > > On Mon, Jan 11, 2021, at 8:07 AM, Christophe Lyon wrote:
> > > > > On Sat, 9 Jan 2021 at 14:09, Christophe Lyon
> > > > > <[email protected]> wrote:
> > > > > >
> > > > > > On Sat, 9 Jan 2021 at 13:27, Daniel Engel <[email protected]>
> > > > > > wrote:
> > > > > > >
> > > > > > > On Thu, Jan 7, 2021, at 4:56 AM, Richard Earnshaw wrote:
> > > > > > > > On 07/01/2021 00:59, Daniel Engel wrote:
> > > > > > > > > --snip--
> > > > > > > > >
> > > > > > > > > On Wed, Jan 6, 2021, at 9:05 AM, Richard Earnshaw wrote:
> > > > > > > > > --snip--
> > > > > > > > >
> > > > > > > > >> - finally, your popcount implementations have data in the
> > > > > > > > >> code segment.
> > > > > > > > >> That's going to cause problems when we have compilation
> > > > > > > > >> options such as
> > > > > > > > >> -mpure-code.
> > > > > > > > >
> > > > > > > > > I am just following the precedent of existing lib1funcs (e.g.
> > > > > > > > > __clz2si).
> > > > > > > > > If this matters, you'll need to point in the right direction
> > > > > > > > > for the
> > > > > > > > > fix. I'm not sure it does matter, since these functions are
> > > > > > > > > PIC anyway.
> > > > > > > >
> > > > > > > > That might be a bug in the clz implementations - Christophe:
> > > > > > > > Any thoughts?
> > > > > > >
> > > > > > > __clzsi2() has test coverage in
> > > > > > > "gcc.c-torture/execute/builtin-bitops-1.c"
> > > > > > Thanks, I'll have a closer look at why I didn't see problems.
> > > > > >
> > > > >
> > > > > So, that's because the code goes to the .text section (as opposed to
> > > > > .text.noread)
> > > > > and does not have the PURECODE flag. The compiler takes care of this
> > > > > when generating code with -mpure-code.
> > > > > And the simulator does not complain because it only checks loads from
> > > > > the segment with the PURECODE flag set.
> > > > >
> > > > This is far out of my depth, but can something like:
> > > >
> > > > ifeq (,$(findstring __symbian__,$(shell $(gcc_compile_bare) -dM -E -
> > > > </dev/null)))
> > > >
> > > > be adapted to:
> > > >
> > > > a) detect the state of the -mpure-code switch, and
> > > > b) pass that flag to the preprocessor?
> > > >
> > > > If so, I can probably fix both the target section and the data usage.
> > > > Just have to add a few instructions to finish unrolling the loop.
> > >
> > > I must confess I never checked libgcc's Makefile deeply before,
> > > but it looks like you can probably detect whether -mpure-code is
> > > part of $CFLAGS.
> > >
> > > However, it might be better to write pure-code-safe code
> > > unconditionally because the toolchain will probably not
> > > be rebuilt with -mpure-code as discussed before.
> > > Or that could mean adding a -mpure-code multilib....
> >
> > I have learned a few things since the last update. I think I know how
> > to get -mpure-code out of CFLAGS and into a macro. However, I have hit
> > something of a wall with testing. I can't seem to compile any flavor of
> > libgcc with CFLAGS_FOR_TARGET="-mpure-code".
> >
> > 1. Configuring --with-multilib-list=rmprofile results in build failure:
> >
> > checking for suffix of object files... configure: error: in
> > `/home/mirdan/gcc-obj/arm-none-eabi/libgcc':
> > configure: error: cannot compute suffix of object files: cannot compile
> > See `config.log' for more details
> >
> > cc1: error: -mpure-code only supports non-pic code on M-profile targets
> >
>
> Yes, I did hit that wall too :-)
>
> Hence what we discussed earlier: the toolchain is not rebuilt with
> -mpure-code.
>
> Note that there are problems in newlib too, but users of -mpure-code seem
> to be able to work around that (eg. using their own startup code and no
> stdlib)
Is there a current side project to solve the makefile problems?
I think I'm back to my original question: If libgcc can't be built
with -mpure-code, and users bypass it completely with -nostdlib, then
why this conversation about pure-code compatibility of __clzsi2() etc?
> > 2. Attempting to filter the multib list results in configuration error.
> > This might have been misguided, but it was something I tried:
> >
> > Error: --with-multilib-list=armv6s-m not supported.
> >
> > Error: --with-multilib-list=mthumb/march=armv6s-m/mfloat-abi=soft not
> > supported
>
> I think only 2 values are supported: aprofile and rmprofile.
It looks like this might require a custom t-* multilib in gcc/config/arm.
> > 3. Attempting to configure a single architecture results in a build error.
> >
> > --with-mode=thumb --with-arch=armv6s-m --with-float=soft
> >
> > checking for suffix of object files... configure: error: in
> > `/home/mirdan/gcc-obj/arm-none-eabi/arm/autofp/v5te/fpu/libgcc':
> > configure: error: cannot compute suffix of object files: cannot compile
> > See `config.log' for more details
> >
> > conftest.c:9:10: fatal error: ac_nonexistent.h: No such file or
> > directory
> > 9 | #include <ac_nonexistent.h>
> > | ^~~~~~~~~~~~~~~~~~
> I never saw that error message, but I never build using --with-arch.
> I do use --with-cpu though.
>
> > This has me wondering whether pure-code in libgcc is a real issue ...
> > If there's a way to build libgcc with -mpure-code, please enlighten me.
> I haven't done so yet. Maybe building the toolchain --with-cpu=cortex-m0
> works?
No luck with that. Same error message as before:
4. --with-mode=thumb --with-arch=armv6s-m --with-float=soft
--with-cpu=cortex-m0
Switch "--with-arch" may not be used with switch "--with-cpu"
5. Then: --with-mode=thumb --with-float=soft --with-cpu=cortex-m0
checking for suffix of object files... configure: error: in
`/home/mirdan/gcc-obj/arm-none-eabi/arm/autofp/v5te/fpu/libgcc':
configure: error: cannot compute suffix of object files: cannot compile
See `config.log' for more details
cc1: error: -mpure-code only supports non-pic code on M-profile targets
6. Finally! --with-float=soft --with-cpu=cortex-m0 --disable-multilib
Once you know this, and read the docs sideways, the previous errors are
all probably "works as designed". But, I can still grumble.
With libgcc compiled with -mpure-code, I can confirm that
'builtin-bitops-1.c' (the test for __clzsi2) passed with libgcc as-is.
I then added the SHF_ARM_PURECODE flag to the libgcc assembly functions
and re-ran the test. Still passed. I then added -mpure-code to
RUNTESTFLAGS and re-ran the test. Still passed. readelf confirmed that
the test program is compiling as expected [1]:
[ 2] .text PROGBITS 0000800c 00800c 003314 00 AXy 0 0
4
Key to Flags:
W (write), A (alloc), X (execute), M (merge), S (strings), I (info),
L (link order), O (extra OS processing required), G (group), T (TLS),
C (compressed), x (unknown), o (OS specific), E (exclude),
y (purecode), p (processor specific)
It was only when I started inserting pure-code test directives into
'builtin-bitops-1.c' that 'make check' began to report errors.
/* { dg-do compile } */
...
/* { dg-options "-mpure-code -mfp16-format=ieee" } */
/* { dg-final { scan-assembler-not
"\\.(float|l\\?double|\d?byte|short|int|long|quad|word)\\s+\[^.\]" } } */
However, for reasons [2] [3] [4] [5], this wasn't actually useful. It's
sufficient to say that there are many reasons that non-pure-code
compatible functions exist in libgcc.
Although I'm not sure how useful this will be in light of the previous
findings, I did take the opportunity with a working compile process to
modify the relevant assembly functions for -mpure-code compatibility.
I can manually disassemble the library and verify correct compilation.
I can manually run a non-pure-code builtin-bitops-1 with a pure-code
library to verify correct execution. But, I don't think your standard
regression suite will be able to exercise the new paths.
The patch is below; you can consider this as 34/33 in the series.
Regards,
Daniel
[1] It's pretty clear that the section flags in libgcc have never really
mattered. When the linker strings all of the used objects together,
the original sections disappear into a single output object. The
compiler controls those flags regardless of what libgcc does.)
[2] The existing pure-code tests are compile-only and cover just the
disassembled 'main.o'. There is no test of a complete executable
and there is no execution/simulation.
[3] While other parts of binutils may understand SHF_ARM_PURECODE, I
don't think the simulator checks section flags or throws exceptions.
[4] builtin-bitops-1 modified this way will always fail due to the array
data definitions (longs, longlongs, etc). GCC can't translate those
to instructions. While the ".data" section would presumably be
readable, scan-assembler-not doesn't know the difference.
[5] Even if the simulator were modified to throw exceptions, this will
continue to fail because _mainCRTStartup uses a literal pool.
> Thanks,
>
> Christophe
>
> > > > > > > The 'clzs' and 'ctz' functions should never have problems.
> > > > > > > -mpure-code
> > > > > > > appears to be valid only when the 'movt' instruction is
> > > > > > > available, which
> > > > > > > means that the 'clz' instruction will also be available, so no
> > > > > > > array loads.
> > > > > > No, -mpure-code is also supported with v6m.
> > > > > >
> > > > > > > Is the -mpure-code state detectable as a preprocessor flag? While
> > > > > > No.
> > > > > >
> > > > > > > 'movw'/'movt' appears to be the canonical solution, I'm not sure
> > > > > > > it
> > > > > > > should be the default just because a processor supports Thumb-2.
> > > > > > >
> > > > > > > Do users wanting to use -mpure-code recompile the toolchain to
> > > > > > > avoid
> > > > > > > constant data in compiled C functions? I don't think this is the
> > > > > > > default for the typical toolchain scripts.
> > > > > > No, users of -mpure-code do not recompile the toolchain.
> > > > > >
> > > > > > --snip --
> > > >
> > > > >
> > >
> >
> > Thanks,
> > Daniel
Add -mpure-code support to the CM0 functions.
gcc/libgcc/ChangeLog:
2021-01-16 Daniel Engel <[email protected]>
Makefile.in (MPURE_CODE): New macro defines __PURE_CODE__.
(gcc_compile): Appended MPURE_CODE.
lib1funcs.S (FUNC_START_SECTION): Set flags for __PURE_CODE__.
clz2.S (__clzsi2): Added -mpure-code compatible instructions.
ctz2.S (__ctzsi2): Same.
popcnt.S (__popcountsi2, __popcountdi2): Same.
diff --git a/libgcc/Makefile.in b/libgcc/Makefile.in
index 2de57519734..cd6b5f9c1b0 100644
--- a/libgcc/Makefile.in
+++ b/libgcc/Makefile.in
@@ -303,6 +303,9 @@ CRTSTUFF_CFLAGS = -O2 $(GCC_CFLAGS) $(INCLUDES)
$(MULTILIB_CFLAGS) -g0 \
# Extra flags to use when compiling crt{begin,end}.o.
CRTSTUFF_T_CFLAGS =
+# Pass the -mpure-code flag into assembly for conditional compilation.
+MPURE_CODE = $(if $(findstring -mpure-code,$(CFLAGS)), -D__PURE_CODE__)
+
MULTIDIR := $(shell $(CC) $(CFLAGS) -print-multi-directory)
MULTIOSDIR := $(shell $(CC) $(CFLAGS) -print-multi-os-directory)
@@ -312,7 +315,7 @@ inst_slibdir = $(slibdir)$(MULTIOSSUBDIR)
gcc_compile_bare = $(CC) $(INTERNAL_CFLAGS)
compile_deps = -MT $@ -MD -MP -MF $(basename $@).dep
-gcc_compile = $(gcc_compile_bare) -o $@ $(compile_deps)
+gcc_compile = $(gcc_compile_bare) -o $@ $(compile_deps) $(MPURE_CODE)
gcc_s_compile = $(gcc_compile) -DSHARED
objects = $(filter %$(objext),$^)
diff --git a/libgcc/config/arm/clz2.S b/libgcc/config/arm/clz2.S
index a2de45ff651..97a44f5d187 100644
--- a/libgcc/config/arm/clz2.S
+++ b/libgcc/config/arm/clz2.S
@@ -214,17 +214,40 @@ FUNC_ENTRY clzsi2
IT(sub,ne) r2, #4
LLSYM(__clz2):
+ #if defined(__PURE_CODE__) && __PURE_CODE__
+ // Without access to table data, continue unrolling the loop.
+ lsrs r1, r0, #2
+
+ #ifdef __HAVE_FEATURE_IT
+ do_it ne,t
+ #else
+ beq LLSYM(__clz1)
+ #endif
+
+ // Out of 4 bits, the first '1' is somewhere in the highest 2,
+ // so the lower 2 bits are no longer interesting.
+ IT(mov,ne) r0, r1
+ IT(sub,ne) r2, #2
+
+ LLSYM(__clz1):
+ // Convert remainder {0,1,2,3} to {0,1,2,2}.
+ lsrs r1, r0, #1
+ bics r0, r1
+
+ #else /* !__PURE_CODE__ */
// Load the remainder by index
adr r1, LLSYM(__clz_remainder)
ldrb r0, [r1, r0]
+ #endif /* !__PURE_CODE__ */
#endif /* !__OPTIMIZE_SIZE__ */
// Account for the remainder.
subs r0, r2, r0
RET
- #if !defined(__OPTIMIZE_SIZE__) || !__OPTIMIZE_SIZE__
+ #if !(defined(__OPTIMIZE_SIZE__) && __OPTIMIZE_SIZE__) && \
+ !(defined(__PURE_CODE__) && __PURE_CODE__)
.align 2
LLSYM(__clz_remainder):
.byte 0,1,2,2,3,3,3,3,4,4,4,4,4,4,4,4
diff --git a/libgcc/config/arm/ctz2.S b/libgcc/config/arm/ctz2.S
index b9528a061a2..6a49d64f3a6 100644
--- a/libgcc/config/arm/ctz2.S
+++ b/libgcc/config/arm/ctz2.S
@@ -209,11 +209,44 @@ FUNC_ENTRY ctzsi2
IT(sub,ne) r2, #4
LLSYM(__ctz2):
+ #if defined(__PURE_CODE__) && __PURE_CODE__
+ // Without access to table data, continue unrolling the loop.
+ lsls r1, r0, #2
+
+ #ifdef __HAVE_FEATURE_IT
+ do_it ne, t
+ #else
+ beq LLSYM(__ctz1)
+ #endif
+
+ // Out of 4 bits, the first '1' is somewhere in the lowest 2,
+ // so the higher 2 bits are no longer interesting.
+ IT(mov,ne) r0, r1
+ IT(sub,ne) r2, #2
+
+ LLSYM(__ctz1):
+ // Convert remainder {0,1,2,3} in $r0[31:30] to {0,2,1,2}.
+ lsrs r0, #31
+
+ #ifdef __HAVE_FEATURE_IT
+ do_it cs, t
+ #else
+ bcc LLSYM(__ctz_zero)
+ #endif
+
+ // If bit[30] of the remainder is set, neither of these bits count
+ // towards the result. Bit[31] must be cleared.
+ // Otherwise, bit[31] becomes the final remainder.
+ IT(sub,cs) r2, #2
+ IT(eor,cs) r0, r0
+
+ #else /* !__PURE_CODE__ */
// Look up the remainder by index.
lsrs r0, #28
adr r3, LLSYM(__ctz_remainder)
ldrb r0, [r3, r0]
+ #endif /* !__PURE_CODE__ */
#endif /* !__OPTIMIZE_SIZE__ */
LLSYM(__ctz_zero):
@@ -221,8 +254,9 @@ FUNC_ENTRY ctzsi2
subs r0, r2, r0
RET
- #if (!defined(__ARM_FEATURE_CLZ) || !__ARM_FEATURE_CLZ) && \
- (!defined(__OPTIMIZE_SIZE__) || !__OPTIMIZE_SIZE__)
+ #if !(defined(__ARM_FEATURE_CLZ) && __ARM_FEATURE_CLZ) && \
+ !(defined(__OPTIMIZE_SIZE__) && __OPTIMIZE_SIZE__) && \
+ !(defined(__PURE_CODE__) && __PURE_CODE__)
.align 2
LLSYM(__ctz_remainder):
.byte 0,4,3,4,2,4,3,4,1,4,3,4,2,4,3,4
diff --git a/libgcc/config/arm/lib1funcs.S b/libgcc/config/arm/lib1funcs.S
index 5148957144b..59b2370e160 100644
--- a/libgcc/config/arm/lib1funcs.S
+++ b/libgcc/config/arm/lib1funcs.S
@@ -454,7 +454,12 @@ SYM (\name):
Use the *_START_SECTION macros for declarations that the linker should
place in a non-defailt section (e.g. ".rodata", ".text.subsection"). */
.macro FUNC_START_SECTION name section
- .section \section,"x"
+#ifdef __PURE_CODE__
+ /* SHF_ARM_PURECODE | SHF_ALLOC | SHF_EXECINSTR */
+ .section \section,"0x20000006",%progbits
+#else
+ .section \section,"ax",%progbits
+#endif
.align 0
FUNC_ENTRY \name
.endm
diff --git a/libgcc/config/arm/popcnt.S b/libgcc/config/arm/popcnt.S
index 51b1ed745ee..d6f65403b5d 100644
--- a/libgcc/config/arm/popcnt.S
+++ b/libgcc/config/arm/popcnt.S
@@ -23,6 +23,29 @@
<http://www.gnu.org/licenses/>. */
+#if defined(L_popcountdi2) || defined(L_popcountsi2)
+
+.macro ldmask reg, temp, value
+ #if defined(__PURE_CODE__) && (__PURE_CODE__)
+ #ifdef NOT_ISA_TARGET_32BIT
+ movs \reg, \value
+ lsls \temp, \reg, #8
+ orrs \reg, \temp
+ lsls \temp, \reg, #16
+ orrs \reg, \temp
+ #else
+ // Assumption: __PURE_CODE__ only support M-profile.
+ movw \reg ((\value) * 0x101)
+ movt \reg ((\value) * 0x101)
+ #endif
+ #else
+ ldr \reg, =((\value) * 0x1010101)
+ #endif
+.endm
+
+#endif
+
+
#ifdef L_popcountdi2
// int __popcountdi2(int)
@@ -49,7 +72,7 @@ FUNC_START_SECTION popcountdi2 .text.sorted.libgcc.popcountdi2
#else /* !__OPTIMIZE_SIZE__ */
// Load the one-bit alternating mask.
- ldr r3, =0x55555555
+ ldmask r3, r2, 0x55
// Reduce the second word.
lsrs r2, r1, #1
@@ -62,7 +85,7 @@ FUNC_START_SECTION popcountdi2 .text.sorted.libgcc.popcountdi2
subs r0, r2
// Load the two-bit alternating mask.
- ldr r3, =0x33333333
+ ldmask r3, r2, 0x33
// Reduce the second word.
lsrs r2, r1, #2
@@ -140,7 +163,7 @@ FUNC_ENTRY popcountsi2
#else /* !__OPTIMIZE_SIZE__ */
// Load the one-bit alternating mask.
- ldr r3, =0x55555555
+ ldmask r3, r2, 0x55
// Reduce the word.
lsrs r1, r0, #1
@@ -148,7 +171,7 @@ FUNC_ENTRY popcountsi2
subs r0, r1
// Load the two-bit alternating mask.
- ldr r3, =0x33333333
+ ldmask r3, r2, 0x33
// Reduce the word.
lsrs r1, r0, #2
@@ -158,7 +181,7 @@ FUNC_ENTRY popcountsi2
adds r0, r1
// Load the four-bit alternating mask.
- ldr r3, =0x0F0F0F0F
+ ldmask r3, r2, 0x0F
// Reduce the word.
lsrs r1, r0, #4
