From 4fd3801e32fee9e25f04ae68a4cbfc95533d56df Mon Sep 17 00:00:00 2001
From: Vitaly Shmagun <65466721+Vitve4@users.noreply.github.com>
Date: Thu, 28 May 2020 20:31:49 +0300
Subject: Improve reading ADC result

7.3.0-atmel3.6.1-arduino7 gcc fails to optimize separate reading from ADCL and ADCH. It produces additionally three eor commands or in some cases two mov commands in the assembly code (see discussion #344). These commands swap register contents before store them to data area. So they are completely unnecessary. Reading ADC result with ADC macro fixes it and gcc generates the right code..
---
 cores/arduino/wiring_analog.c | 18 +++++-------------
 1 file changed, 5 insertions(+), 13 deletions(-)

(limited to 'cores/arduino')

diff --git a/cores/arduino/wiring_analog.c b/cores/arduino/wiring_analog.c
index e237d6d..0de64f7 100644
--- a/cores/arduino/wiring_analog.c
+++ b/cores/arduino/wiring_analog.c
@@ -37,7 +37,6 @@ void analogReference(uint8_t mode)
 
 int analogRead(uint8_t pin)
 {
-	uint8_t low, high;
 
 #if defined(analogPinToChannel)
 #if defined(__AVR_ATmega32U4__)
@@ -74,27 +73,20 @@ int analogRead(uint8_t pin)
 	// without a delay, we seem to read from the wrong channel
 	//delay(1);
 
-#if defined(ADCSRA) && defined(ADCL)
+#if defined(ADCSRA) && defined(ADC)
 	// start the conversion
 	sbi(ADCSRA, ADSC);
 
 	// ADSC is cleared when the conversion finishes
 	while (bit_is_set(ADCSRA, ADSC));
 
-	// we have to read ADCL first; doing so locks both ADCL
-	// and ADCH until ADCH is read.  reading ADCL second would
-	// cause the results of each conversion to be discarded,
-	// as ADCL and ADCH would be locked when it completed.
-	low  = ADCL;
-	high = ADCH;
+	// ADC macro takes care of reading ADC register.
+	// avr-gcc implements the proper reading order: ADCL is read first.
+	return ADC;
 #else
 	// we dont have an ADC, return 0
-	low  = 0;
-	high = 0;
+	return 0;
 #endif
-
-	// combine the two bytes
-	return (high << 8) | low;
 }
 
 // Right now, PWM output only works on the pins with
-- 
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