[PATCH] RISC-V: Adjust __global_pointer$ value to reduce code size.

Jim Wilson jimw@sifive.com
Mon Oct 15 23:01:00 GMT 2018


For RISC-V, the compiler doesn't emit gp-relative references, they are created
during relaxation.  Before this patch, the gp value is sdata+0x800 which is
guaranteed to cover as much as sdata+sbss as possible, but when they are
smaller than 0x1000 we end up wasting some of the gp-relative range.  This
changes the calculation to shift gp down when possible, while still covering
all of sdata+sbss.  This reduces code size for a trivial hello world program
by about 0.5%.  For a more realistic benchmark set, EEMBC Automotive-1.1, I'm
seeing about 0.1% code size reduction on average.  For large programs, this
has no effect.

This was tested with riscv{32,64}-{elf,linux} cross binutils build and check,
there were no regressions.  I did have to update one linker testcase to disable
relaxation because it assumed that variable accesses in data would not be
relaxed.  It was also tested with riscv32-elf and riscv64-linux cross gcc
build and check, and a linux+buildroot build and boot.

Committed.

Jim

	ld/
	* emulparams/elf32lriscv-defs.sh (DATA_START_SYMBOLS): New.
	(SDATA_START_SYMBOLS): Define __SDATA_BEGIN__.  Don't define
	__global_pointer$.
	(OTHER_END_SYMBOLS): New.  Define __global_pointer$.
	* testsuite/ld-riscv-elf/pcrel-lo-addend-2.d (#ld): Add --no-relax.
---
 ld/emulparams/elf32lriscv-defs.sh             | 13 ++++++++++++-
 ld/testsuite/ld-riscv-elf/pcrel-lo-addend-2.d |  2 +-
 2 files changed, 13 insertions(+), 2 deletions(-)

diff --git a/ld/emulparams/elf32lriscv-defs.sh b/ld/emulparams/elf32lriscv-defs.sh
index 91015d4471..5ac3b6023d 100644
--- a/ld/emulparams/elf32lriscv-defs.sh
+++ b/ld/emulparams/elf32lriscv-defs.sh
@@ -30,8 +30,19 @@ TEXT_START_ADDR=0x10000
 MAXPAGESIZE="CONSTANT (MAXPAGESIZE)"
 COMMONPAGESIZE="CONSTANT (COMMONPAGESIZE)"
 
-SDATA_START_SYMBOLS="${CREATE_SHLIB-__global_pointer$ = . + 0x800;}
+DATA_START_SYMBOLS="${CREATE_SHLIB-__DATA_BEGIN__ = .;}"
+
+SDATA_START_SYMBOLS="${CREATE_SHLIB-__SDATA_BEGIN__ = .;}
     *(.srodata.cst16) *(.srodata.cst8) *(.srodata.cst4) *(.srodata.cst2) *(.srodata .srodata.*)"
 
 INITIAL_READONLY_SECTIONS=".interp         : { *(.interp) } ${CREATE_PIE-${INITIAL_READONLY_SECTIONS}}"
 INITIAL_READONLY_SECTIONS="${RELOCATING+${CREATE_SHLIB-${INITIAL_READONLY_SECTIONS}}}"
+
+# We must cover as much of sdata as possible if it exists.  If sdata+bss is
+# smaller than 0x1000 then we should start from bss end to cover as much of
+# the program as possible.  But we can't allow gp to cover any of rodata, as
+# the address of variables in rodata may change during relaxation, so we start
+# from data in that case.
+OTHER_END_SYMBOLS="${CREATE_SHLIB-__BSS_END__ = .;
+    __global_pointer$ = MIN(__SDATA_BEGIN__ + 0x800,
+		            MAX(__DATA_BEGIN__ + 0x800, __BSS_END__ - 0x800));}"
diff --git a/ld/testsuite/ld-riscv-elf/pcrel-lo-addend-2.d b/ld/testsuite/ld-riscv-elf/pcrel-lo-addend-2.d
index 9e94c5c399..039de102c3 100644
--- a/ld/testsuite/ld-riscv-elf/pcrel-lo-addend-2.d
+++ b/ld/testsuite/ld-riscv-elf/pcrel-lo-addend-2.d
@@ -1,5 +1,5 @@
 #name: %pcrel_lo overflow with an addend
 #source: pcrel-lo-addend-2.s
 #as: -march=rv32ic
-#ld: -melf32lriscv
+#ld: -melf32lriscv --no-relax
 #error: .*dangerous relocation: %pcrel_lo overflow with an addend
-- 
2.17.1



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