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Re: [Qemu-devel] [PATCH 03/14] cg3: remove unused width and height varia

From: Richard Henderson
Subject: Re: [Qemu-devel] [PATCH 03/14] cg3: remove unused width and height variables
Date: Tue, 18 Apr 2017 07:55:41 -0700
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.8.0

On 04/18/2017 07:38 AM, Mark Cave-Ayland wrote:
On 15/04/17 11:54, Richard Henderson wrote:

On 04/05/2017 01:35 AM, Mark Cave-Ayland wrote:
These aren't required since we can use the display width and height

Signed-off-by: Mark Cave-Ayland <address@hidden>
 hw/display/cg3.c |   15 ++++++---------
 1 file changed, 6 insertions(+), 9 deletions(-)

diff --git a/hw/display/cg3.c b/hw/display/cg3.c
index b42f60e..178a6dd 100644
--- a/hw/display/cg3.c
+++ b/hw/display/cg3.c
@@ -93,14 +93,11 @@ static void cg3_update_display(void *opaque)
     uint32_t *data;
     uint32_t dval;
     int x, y, y_start;
-    unsigned int width, height;
     ram_addr_t page, page_min, page_max;

     if (surface_bits_per_pixel(surface) != 32) {
-    width = s->width;
-    height = s->height;

     y_start = -1;
     page_min = -1;
@@ -110,11 +107,11 @@ static void cg3_update_display(void *opaque)
     data = (uint32_t *)surface_data(surface);

-    for (y = 0; y < height; y++) {
+    for (y = 0; y < s->height; y++) {

I suspect the generated code is much worse, since the compiler can no
longer tell that the loop bounds are constant.

You probably do want to keep width and height in local variables across
this function.

Can you explain a bit more about this? My thoughts were that with
optimisation enabled the compiler would assume s->width is constant
throughout the loop by default (or at least in OpenBIOS I've had to
explicitly mark variables as volatile to ensure the compiler refetches
the original value instead of reusing a previous copy from the

With data in memory, as for foo->bar, the compiler is obliged to consider the memory may be changed for e.g. an intervening function call baz(), when foo is "global", as it is here. You have many of those in those loops.


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