From: Eric Botcazou <ebotcazou@adacore.com>
To: gcc-patches@gcc.gnu.org
Subject: Small tweak to RTL expansion of some array accesses on RISC targets
Date: Thu, 29 Jun 2017 19:23:00 -0000 [thread overview]
Message-ID: <10060997.SGyfb6a1MB@polaris> (raw)
[-- Attachment #1: Type: text/plain, Size: 1909 bytes --]
I noticed that, when a variable-sized object declared on the stack turns out
to be of fixed size, the optimizer can replace the call to __builtin_alloca by
the declaration of fixed-size local array. Now, even if the alignment of the
object is explicitly preserved, the alignment of its type is not since the
type of the local array is always unsigned char.
On RISC targets (STRICT_ALIGNMENT / SLOW_UNALIGNED_ACCESS to be precise), this
causes any read larger than unsigned char to go through the bitfield expansion
circuitry, because expand_expr_real_1 has:
/* If the field isn't aligned enough to fetch as a memref,
fetch it as a bit field. */
|| (mode1 != BLKmode
&& (((TYPE_ALIGN (TREE_TYPE (tem)) < GET_MODE_ALIGNMENT (mode)
|| (bitpos % GET_MODE_ALIGNMENT (mode) != 0)
|| (MEM_P (op0)
&& (MEM_ALIGN (op0) < GET_MODE_ALIGNMENT (mode1)
|| (bitpos % GET_MODE_ALIGNMENT (mode1) != 0))))
&& modifier != EXPAND_MEMORY
&& ((modifier == EXPAND_CONST_ADDRESS
|| modifier == EXPAND_INITIALIZER)
? STRICT_ALIGNMENT
: SLOW_UNALIGNED_ACCESS (mode1, MEM_ALIGN (op0))))
In other words, even if the alignment of the MEM is sufficient (and it is),
the test on TYPE_ALIGN (TREE_TYPE (tem)) is true since TREE_TYPE (tem) is
unsigned char.
I think that the test on TYPE_ALIGN (TREE_TYPE (tem)) is superfluous when op0
is a MEM because the second part of the test is more precise and sufficient,
so the attached patchlet uses a conditional expression to implement that.
Bootstrapped/regtested on SPARC/Solaris, SPARC64/Linux, PowerPC64/Linux and
Aarch64/Linux, applied on the mainline as obvious.
2017-06-29 Eric Botcazou <ebotcazou@adacore.com>
* expr.c (expand_expr) <normal_inner_ref>: When testing for unaligned
objects, take into account only the alignment of 'op0' and 'mode1' if
'op0' is a MEM.
--
Eric Botcazou
[-- Attachment #2: p.diff --]
[-- Type: text/x-patch, Size: 976 bytes --]
Index: expr.c
===================================================================
--- expr.c (revision 249619)
+++ expr.c (working copy)
@@ -10631,11 +10631,11 @@ expand_expr_real_1 (tree exp, rtx target
/* If the field isn't aligned enough to fetch as a memref,
fetch it as a bit field. */
|| (mode1 != BLKmode
- && (((TYPE_ALIGN (TREE_TYPE (tem)) < GET_MODE_ALIGNMENT (mode)
- || (bitpos % GET_MODE_ALIGNMENT (mode) != 0)
- || (MEM_P (op0)
- && (MEM_ALIGN (op0) < GET_MODE_ALIGNMENT (mode1)
- || (bitpos % GET_MODE_ALIGNMENT (mode1) != 0))))
+ && (((MEM_P (op0)
+ ? MEM_ALIGN (op0) < GET_MODE_ALIGNMENT (mode1)
+ || (bitpos % GET_MODE_ALIGNMENT (mode1) != 0)
+ : TYPE_ALIGN (TREE_TYPE (tem)) < GET_MODE_ALIGNMENT (mode)
+ || (bitpos % GET_MODE_ALIGNMENT (mode) != 0))
&& modifier != EXPAND_MEMORY
&& ((modifier == EXPAND_CONST_ADDRESS
|| modifier == EXPAND_INITIALIZER)
reply other threads:[~2017-06-29 19:23 UTC|newest]
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