[PATCH v4 09/11] s390: Store SFrame CFA offset adjusted and scaled down
Jens Remus
jremus@linux.ibm.com
Mon Jul 7 16:21:50 GMT 2025
In SFrame V2 the size of the offsets following an SFrame FRE can be
either signed 8-bit, 16-bit, or 32-bit integer, with the largest offset
determining their size:
1. CFA offset from CFA base register
2. RA (stack save slot) offset from CFA, usually -48 on s390x if saved
3. FP (stack save slot) offset from CFA, usually -72 on s390x if saved
The FP and RA offsets from CFA, when FP/RA saved on the stack, usually
have fixed values that fit into signed 8-bit SFrame offsets. Likewise
the DWARF register numbers on s390x of general registers (GR; 0-15) and
floating-point registers (FPR; 16-31), when FP/RA saved in registers.
With that the CFA offset from CFA base register has the greatest impact
on the signed SFrame offset size.
The s390x ELF ABI defines the stack pointer (SP) to be 8-byte aligned
[1] and the CFA as SP at call site + 160 [2]. The CFA offset from CFA
base register is therefore always a multiple of 8.
On s390x store the SFrame CFA offset from CFA base register scaled down
by the s390x-specific CFA alignment factor of 8, in addition to the
adjustment by the s390x-specific CFA adjustment of -160, to further
improve the use of signed 8-bit SFrame offsets. This is similar to the
DWARF data alignment factor getting factored out from certain offsets
stored in DWARF CFI.
[1]: s390x ELF ABI, sections "Register Roles" and "Stack Frame
Allocation", https://github.com/IBM/s390x-abi/releases
[2]: s390x ELF ABI, commit 4e38ad9c8a88 ("Document the CFA"),
https://github.com/IBM/s390x-abi/commit/4e38ad9c8a88
include/
* sframe.h (SFRAME_S390X_CFA_OFFSET_ALIGNMENT_FACTOR): Define
s390x-specific CFA offset alignment factor.
(SFRAME_V2_FRE_S390X_CFA_OFFSET_ENCODE,
SFRAME_V2_FRE_S390X_CFA_OFFSET_DECODE): Scale down/up by
SFRAME_S390X_CFA_OFFSET_ALIGNMENT_FACTOR.
libsframe/
* doc/sframe-spec.texi (s390x,
SFRAME_S390X_CFA_OFFSET_ALIGNMENT_FACTOR): Document s390x-
specific CFA offset alignment factor.
Signed-off-by: Jens Remus <jremus@linux.ibm.com>
---
Notes (jremus):
Changes in V4:
- Indent list of s390x-specific changes in SFrame spec.
- Fix typo in commit message ("with"). (Indu)
- Use comma after "i.e." in SFrame spec. (Indu)
- Use "revert" instead of "unapply" in SFrame spec. (Indu)
Changes in V3:
- Update "Changes from Version 1 to Version 2" in SFrame spec. (Indu)
- Regenerate statistics in patch notes below. Differences to V2 due
to change in min/max N-bit offset accounting (was unusual in V1/V2)
and use in newer GCC.
Changes in V2:
- Reword commit message (s390 to s390x, offsets, FDE to FRE). (Indu)
- Rename s390x-specific macros from *_S390_* to *_S390X_*. (Indu)
- Use "s390x" instead of "S390" in comments and SFrame spec. (Indu)
- Change order of CFA restore instructions in SFrame spec. (Indu)
A test build of Glibc tag 2.41 on s390x libc.so shows an additional ~9%
reduction in .sframe section size due to storing the SFrame CFA offsets
scaled down by 8 in addition to the adjustment by -160. In total
adjusting and scaling down reduces the .sframe size by ~17%. The
overall number of offsets larger than 8-bit is effectively reduced down
to ~2.4%.
Statistics for libc.so - base (no CFA offset adjustment nor scaling):
.sframe size: 169,697 bytes
VALUE TOTAL MIN MAX AVG
FDEs: 3651 - - -
FREs/FDE: 15237 1 20 4
Offsets/FDE: 29795 1 38 8
8-bit: 0 0 0 0
16-bit: 29795 1 38 8
32-bit: 0 0 0 0
Offsets/FRE: 29795 1 3 1
8-bit: - 0 0 0
16-bit: - 1 3 1
32-bit: - 0 0 0
O_Padd/FDE: 345 - - 0
8-bit: 0
16-bit: 345
32-bit: 0
Statistics for libc.so - CFA offset adjustment (no scaling):
.sframe size: 154,698 bytes
VALUE TOTAL MIN MAX AVG
FDEs: 3651 - - -
FREs/FDE: 15237 1 20 4
Offsets/FDE: 29795 1 38 8
8-bit: 14999 1 38 4
16-bit: 14796 0 21 4
32-bit: 0 0 0 0
Offsets/FRE: 29795 1 3 1
8-bit: - 1 3 0
16-bit: - 1 3 0
32-bit: - 0 0 0
O_Padd/FDE: 345 - - 0
8-bit: 286
16-bit: 59
32-bit: 0
Statistics for libc.so - CFA offset adjustment and scaling:
.sframe size: 140,604 bytes
VALUE TOTAL MIN MAX AVG
FDEs: 3651 - - -
FREs/FDE: 15237 1 20 4
Offsets/FDE: 29795 1 38 8
8-bit: 29093 1 38 7
16-bit: 702 0 15 0
32-bit: 0 0 0 0
Offsets/FRE: 29795 1 3 1
8-bit: - 1 3 1
16-bit: - 2 3 0
32-bit: - 0 0 0
O_Padd/FDE: 345 - - 0
8-bit: 345
16-bit: 0
32-bit: 0
include/sframe.h | 11 ++++++++---
libsframe/doc/sframe-spec.texi | 23 ++++++++++++++---------
2 files changed, 22 insertions(+), 12 deletions(-)
diff --git a/include/sframe.h b/include/sframe.h
index db137393532e..1b967d405d2e 100644
--- a/include/sframe.h
+++ b/include/sframe.h
@@ -377,12 +377,17 @@ typedef struct sframe_frame_row_entry_addr4
(1ULL << ((SFRAME_FRE_TYPE_ADDR4 * 2) * 8))
/* On s390x, the CFA offset from CFA base register is by definition a minimum
- of 160. Store it adjusted by -160 to enable use of 8-bit SFrame offsets. */
+ of 160. Store it adjusted by -160 to enable use of 8-bit SFrame offsets.
+ Additionally scale by an alignment factor of 8, as the SP and thus CFA
+ offset on s390x is always 8-byte aligned. */
#define SFRAME_S390X_CFA_OFFSET_ADJUSTMENT SFRAME_S390X_SP_VAL_OFFSET
+#define SFRAME_S390X_CFA_OFFSET_ALIGNMENT_FACTOR 8
#define SFRAME_V2_FRE_S390X_CFA_OFFSET_ENCODE(offset) \
- ((offset) + SFRAME_S390X_CFA_OFFSET_ADJUSTMENT)
+ (((offset) + SFRAME_S390X_CFA_OFFSET_ADJUSTMENT) \
+ / SFRAME_S390X_CFA_OFFSET_ALIGNMENT_FACTOR)
#define SFRAME_V2_FRE_S390X_CFA_OFFSET_DECODE(offset) \
- ((offset) - SFRAME_S390X_CFA_OFFSET_ADJUSTMENT)
+ (((offset) * SFRAME_S390X_CFA_OFFSET_ALIGNMENT_FACTOR) \
+ - SFRAME_S390X_CFA_OFFSET_ADJUSTMENT)
/* On s390x, the CFA is defined as SP at call site + 160. Therefore the
SP value offset from CFA is -160. */
diff --git a/libsframe/doc/sframe-spec.texi b/libsframe/doc/sframe-spec.texi
index 9814c975147d..0e79cbd6d6b7 100644
--- a/libsframe/doc/sframe-spec.texi
+++ b/libsframe/doc/sframe-spec.texi
@@ -157,10 +157,12 @@ SFRAME_CFA_FIXED_RA_INVALID). Used on s390x as padding offset to represent
FP without RA saved.
@item SFRAME_S390X_CFA_OFFSET_ADJUSTMENT: CFA offset (from CFA base register)
adjustment value. Used to enable use of 8-bit SFrame offsets on s390x.
+ @item SFRAME_S390X_CFA_OFFSET_ALIGNMENT_FACTOR: CFA offset alignment factor.
+Used to scale down the CFA offset to improve the use of 8-bit SFrame offsets.
@item SFRAME_V2_FRE_S390X_CFA_OFFSET_ENCODE: Encode CFA offset (i.e., apply
-CFA offset adjustment).
- @item SFRAME_V2_FRE_S390X_CFA_OFFSET_DECODE: Decode CFA offset (i.e., revert
-CFA offset adjustment).
+CFA offset adjustment and then scale down by CFA offset alignment factor).
+ @item SFRAME_V2_FRE_S390X_CFA_OFFSET_DECODE: Decode CFA offset (i.e., scale up
+by CFA offset alignment factor and then revert CFA offset adjustment).
@end itemize
@end itemize
@@ -892,14 +894,17 @@ in any stack frame.
Irrespective of the ABI, the first stack offset is always used to locate the
CFA. On s390x the value of the offset is stored adjusted by the s390x-specific
-@code{SFRAME_S390X_CFA_OFFSET_ADJUSTMENT} to enable the use of signed 8-bit
-offsets on s390x.
+@code{SFRAME_S390X_CFA_OFFSET_ADJUSTMENT} and scaled down by the s390x-specific
+@code{SFRAME_S390X_CFA_OFFSET_ALIGNMENT_FACTOR}, to enable and improve the use
+of signed 8-bit offsets on s390x.
s390x-specific helpers @code{SFRAME_V2_FRE_S390X_CFA_OFFSET_ENCODE} and
-@code{SFRAME_V2_FRE_S390X_CFA_OFFSET_DECODE} are provided to perform and undo
-the adjustment. The CFA offset can therefore be interpreted as:
-CFA = @code{BASE_REG} + @code{SFRAME_V2_FRE_S390X_CFA_OFFSET_DECODE(offset1)}
+@code{SFRAME_V2_FRE_S390X_CFA_OFFSET_DECODE} are provided to perform or undo
+the adjustment and scaling. The CFA offset can therefore be interpreted as:
+CFA = @code{BASE_REG} + offset1 - @code{SFRAME_S390X_CFA_OFFSET_ADJUSTMENT}
or
-CFA = @code{BASE_REG} + offset1 - @code{SFRAME_S390X_CFA_OFFSET_ADJUSTMENT}.
+CFA = @code{BASE_REG}
+ + (offset1 * @code{SFRAME_S390X_CFA_OFFSET_ALIGNMENT_FACTOR})
+ - @code{SFRAME_S390X_CFA_OFFSET_ADJUSTMENT}.
The identification of the @code{BASE_REG} is done by using the
@code{fre_cfa_base_reg_id} field in the SFrame FRE info word.
--
2.48.1
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