From ae85ad3a95d6df3c4131d02fd327809a29d10b33 Mon Sep 17 00:00:00 2001 From: Wilco Dijkstra Date: Tue, 5 Sep 2017 13:27:58 +0000 Subject: [PATCH] Improve alloca alignment This patch improves alloca alignment. Currently alloca reserves too much space as it aligns twice, and generates unnecessary stack alignment code. When the requested alignment is lower than the stack alignment, no extra alignment is needed. If the requested alignment is higher, we need to increase the size by the difference of the requested alignment and the stack alignment. As a result, the alloca alignment is exactly as expected: alloca (16): sub sp, sp, #16 mov x1, sp alloca (x): add x0, x0, 15 and x0, x0, -16 sub sp, sp, x0 mov x0, sp __builtin_alloca_with_align (x, 512): add x0, x0, 63 and x0, x0, -16 sub sp, sp, x0 add x0, sp, 63 and x0, x0, -64 gcc/ * explow.c (get_dynamic_stack_size): Improve dynamic alignment. From-SVN: r251713 --- gcc/ChangeLog | 4 ++++ gcc/explow.c | 21 +++++++++++++-------- 2 files changed, 17 insertions(+), 8 deletions(-) diff --git a/gcc/ChangeLog b/gcc/ChangeLog index 7f5b384bca4..58d162972ba 100644 --- a/gcc/ChangeLog +++ b/gcc/ChangeLog @@ -1,3 +1,7 @@ +2017-09-05 Wilco Dijkstra + + * explow.c (get_dynamic_stack_size): Improve dynamic alignment. + 2017-09-05 Richard Biener PR tree-optimization/82084 diff --git a/gcc/explow.c b/gcc/explow.c index 13736a586d8..638dc5f8f0f 100644 --- a/gcc/explow.c +++ b/gcc/explow.c @@ -1240,15 +1240,20 @@ get_dynamic_stack_size (rtx *psize, unsigned size_align, example), so we must preventively align the value. We leave space in SIZE for the hole that might result from the alignment operation. */ - extra = (required_align - BITS_PER_UNIT) / BITS_PER_UNIT; - size = plus_constant (Pmode, size, extra); - size = force_operand (size, NULL_RTX); - - if (flag_stack_usage_info && pstack_usage_size) - *pstack_usage_size += extra; + /* Since the stack is presumed to be aligned before this allocation, + we only need to increase the size of the allocation if the required + alignment is more than the stack alignment. */ + if (required_align > STACK_BOUNDARY) + { + extra = (required_align - STACK_BOUNDARY) / BITS_PER_UNIT; + size = plus_constant (Pmode, size, extra); + size = force_operand (size, NULL_RTX); + if (size_align > STACK_BOUNDARY) + size_align = STACK_BOUNDARY; - if (extra && size_align > BITS_PER_UNIT) - size_align = BITS_PER_UNIT; + if (flag_stack_usage_info && pstack_usage_size) + *pstack_usage_size += extra; + } /* Round the size to a multiple of the required stack alignment. Since the stack is presumed to be rounded before this allocation, -- 2.30.2