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Re: New option -flimit-function-alignment
On 10/12/2016 09:27 PM, Denys Vlasenko wrote:
Yes, something like "if max_skip >= func_size, temporarily lower
max_skip to func_size-1" (because otherwise we can create padding
bigger-or-equal to the entire function in size, which is stupid
- it's better to just put the function in that space).
This would be a nice.
That would be this patch. Bootstrapped and tested on x86_64-linux, ok?
Bernd
gcc/
* common.opt (flimit-function-alignment): New.
* doc/invoke.texi (-flimit-function-alignment): Document.
* emit-rtl.h (struct rtl_data): Add max_insn_address field.
* final.c (shorten_branches): Set it.
* varasm.c (assemble_start_function): Limit alignment if
requested.
gcc/testsuite/
* gcc.target/i386/align-limit.c: New test.
Index: gcc/common.opt
===================================================================
--- gcc/common.opt (revision 240861)
+++ gcc/common.opt (working copy)
@@ -906,6 +906,9 @@ Align the start of functions.
falign-functions=
Common RejectNegative Joined UInteger Var(align_functions)
+flimit-function-alignment
+Common Report Var(flag_limit_function_alignment) Optimization Init(0)
+
falign-jumps
Common Report Var(align_jumps,0) Optimization UInteger
Align labels which are only reached by jumping.
Index: gcc/doc/invoke.texi
===================================================================
--- gcc/doc/invoke.texi (revision 240861)
+++ gcc/doc/invoke.texi (working copy)
@@ -368,7 +368,7 @@ Objective-C and Objective-C++ Dialects}.
-fno-ira-share-spill-slots @gol
-fisolate-erroneous-paths-dereference -fisolate-erroneous-paths-attribute @gol
-fivopts -fkeep-inline-functions -fkeep-static-functions @gol
--fkeep-static-consts -flive-range-shrinkage @gol
+-fkeep-static-consts -flimit-function-alignment -flive-range-shrinkage @gol
-floop-block -floop-interchange -floop-strip-mine @gol
-floop-unroll-and-jam -floop-nest-optimize @gol
-floop-parallelize-all -flra-remat -flto -flto-compression-level @gol
@@ -8262,6 +8262,12 @@ If @var{n} is not specified or is zero,
Enabled at levels @option{-O2}, @option{-O3}.
+@item -flimit-function-alignment
+If this option is enabled, the compiler tries to avoid unnecessarily
+overaligning functions. It attempts to instruct the assembler to align
+by the amount specified by @option{-falign-functions}, but not to
+skip more bytes than the size of the function.
+
@item -falign-labels
@itemx -falign-labels=@var{n}
@opindex falign-labels
Index: gcc/emit-rtl.h
===================================================================
--- gcc/emit-rtl.h (revision 240861)
+++ gcc/emit-rtl.h (working copy)
@@ -284,6 +284,9 @@ struct GTY(()) rtl_data {
to eliminable regs (like the frame pointer) are set if an asm
sets them. */
HARD_REG_SET asm_clobbers;
+
+ /* The highest address seen during shorten_branches. */
+ int max_insn_address;
};
#define return_label (crtl->x_return_label)
Index: gcc/final.c
===================================================================
--- gcc/final.c (revision 240861)
+++ gcc/final.c (working copy)
@@ -1462,7 +1462,7 @@ shorten_branches (rtx_insn *first)
if (!increasing)
break;
}
-
+ crtl->max_insn_address = insn_current_address;
free (varying_length);
}
Index: gcc/testsuite/gcc.target/i386/align-limit.c
===================================================================
--- gcc/testsuite/gcc.target/i386/align-limit.c (nonexistent)
+++ gcc/testsuite/gcc.target/i386/align-limit.c (working copy)
@@ -0,0 +1,9 @@
+/* { dg-do compile } */
+/* { dg-options "-O2 -falign-functions=64 -flimit-function-alignment" } */
+/* { dg-final { scan-assembler ".p2align 6,,1" } } */
+/* { dg-final { scan-assembler-not ".p2align 6,,63" } } */
+
+void
+test_func (void)
+{
+}
Index: gcc/varasm.c
===================================================================
--- gcc/varasm.c (revision 240861)
+++ gcc/varasm.c (working copy)
@@ -1791,9 +1791,14 @@ assemble_start_function (tree decl, cons
&& align_functions_log > align
&& optimize_function_for_speed_p (cfun))
{
+ int align_log = align_functions_log;
+ int max_skip = align_functions - 1;
+ if (flag_limit_function_alignment && crtl->max_insn_address > 0
+ && max_skip >= crtl->max_insn_address)
+ max_skip = crtl->max_insn_address - 1;
+
#ifdef ASM_OUTPUT_MAX_SKIP_ALIGN
- ASM_OUTPUT_MAX_SKIP_ALIGN (asm_out_file,
- align_functions_log, align_functions - 1);
+ ASM_OUTPUT_MAX_SKIP_ALIGN (asm_out_file, align_log, max_skip);
#else
ASM_OUTPUT_ALIGN (asm_out_file, align_functions_log);
#endif