Control Flow. September 2, Indiana University. Geoffrey Brown, Bryce Himebaugh 2015 September 2, / 21

Transcription

1 Control Flow Geoffrey Brown Bryce Himebaugh Indiana University September 2, 2016 Geoffrey Brown, Bryce Himebaugh 2015 September 2, / 21

2 Outline Condition Codes C Relational Operations C Logical Operations C Control Flow Conditional Code While Loops For Loops Switch Statements Procedure Calls Geoffrey Brown, Bryce Himebaugh 2015 September 2, / 21

3 Control Flow While most readers will be familiar with the C control-flow operations such as if-else, while, until, and for; C has another, frequently disparaged, control-flow statement goto which has the form: goto label:.. label: statement; The label can be anywhere within the same procedure and may be any text that is not a C keyword. Geoffrey Brown, Bryce Himebaugh 2015 September 2, / 21

4 GOTO Example Suppose that we wish to find the minimum of two numbers x and y. We could write this in C as if (x > y) min = y; else min = x; We could rewrite this as if (x > y) goto THEN; min = x; goto END; THEN: min = y; END: Geoffrey Brown, Bryce Himebaugh 2015 September 2, / 21

5 GOTO Example (optimization) Because branches are expensive, it is actually better to do this: END: min = y; if (x > y) goto END; min = x; Geoffrey Brown, Bryce Himebaugh 2015 September 2, / 21

6 The Cortex-M0 instruction set provides a conditional branch instruction that enables the implementation of such operations. The branch instruction has the form: b<c> <label if (c) goto <label > Where c is a condition (the empty condition means always ) and the label is a program location following the current instruction. For example, we can implement the min code as follows. Assume that r0=y and r1=x, and at the end r0=min; we can write this code fragment in the Cortex-M0 instruction set min == r0, r0 == evaluate condition ( postponed) bgt. if (x > y) goto. END movs r0, min = x Geoffrey Brown, Bryce Himebaugh 2015 September 2, / 21

7 In the preceding example, we deferred discussion of the branch condition. Briefly, the condition is computed from the status flags set by preceding instructions. For example,.end: subs r2, r1, r0 bgt. END movs r0, r2 = x - if (x > y) goto. min = x Geoffrey Brown, Bryce Himebaugh 2015 September 2, / 21

8 Comparison Instructions In the preceeding example, r2 was used to store a result that is not needed (only the flag values are used). There are three instructions that are provided to evaluate expressions where only the side effects are preserved cmp and cmn, which perform comparisons, and tst which performs a bit test. cmp is equivalent to subtract and cmn to addition, but without storing the result. cmn Rn, Rm Rn + Rm cmp Rn, Rm Rn - Rm cmp Rn, imm8 Rn - imm8 tst Rn, Rm Rn & Rm Thus we can rewrite our example to: cmp r1, r0 bgt. END movs r0, x - if (x > y) goto. min = x Geoffrey Brown, Bryce Himebaugh 2015 September 2, / 21

9 Condition Codes Suffix Flags Meaning eq Z = 1 Equal, last result was zero ne Z = 0 Not equal, last result was not zero cs or hs C = 1 Higher or same, unsigned cc or lo C = 0 Lower, unsigned mi N = 1 Negative pl N = 0 Plus or zero vs V = 1 Overflow vc V = 0 No overflow hi C = 1 and Z = 0 Higher, unsigned ls C = 0 or Z = 1 Lower or same, unsigned ge N = V Greater than or equal, signed lt N!= V Less than, signed gt Z = 0 and N = V Greater than, signed le Z = 1 and N!= V Less than or equal, signed al - Always, this is the default when no suffix is specified Geoffrey Brown, Bryce Himebaugh 2015 Condition Codes September 2, / 21

10 Logical Value of an Expression Recall that in C, the logical value of an expression is True if the value of the expression is non-zero and False otherwise. For arithmetic expressions, the logical value is captured by the Z status flag as a side effect of expression evaluation. For example, we might decrement a counter and test if the result is zero as: next:... if (-- counter) goto next; Which can be realized in assembly as next: subs r0, 1 bne next Geoffrey Brown, Bryce Himebaugh 2015 C Relational Operations September 2, / 21

11 C Relational Operators C defines a complete set of relational operators == Equal to.!= Not equal to. > Greater than. < Less than. >= Greater than or equal to. <= Less than or equal to. Geoffrey Brown, Bryce Himebaugh 2015 C Relational Operations September 2, / 21

12 C Relational Operators cont. It is not surprising that all of these can be realized in assembly as variations of conditional branches. Returning to the previous example, but changing the condition: next:... if (-- counter >= 0) goto next; Which can be realized in assembly as next: subs r0, 1 bge next Geoffrey Brown, Bryce Himebaugh 2015 C Relational Operations September 2, / 21

13 Generalized Relations More generally, we might want to compare two expressions. For example x <= y. We do this by computing the two operands r0 = r1 = y movs r2, default result is 0 cmps r0, x - y blt x < y? skip next statement movs r2, r2 = 1 skip: Here we have stored the result of the relational operation in r2, which is necessary if the flag values aren t used immediately. In summary, we compute the relational operators either as a side effect of an arithmetic operation (when comparing with 0), or by an explicit comparison of the two operands of the relation. We can used the result immediately in a conditional branch possibly using the branch to store the result required by C (a 0 or 1). Geoffrey Brown, Bryce Himebaugh 2015 C Relational Operations September 2, / 21

14 Conditional Code if (<cond >) { <then -body > } else { <else -body > } <after > if (<cond >) goto THEN: <else -body > goto L1; THEN: <then -body > L1: <after > Geoffrey Brown, Bryce Himebaugh 2015 C Control Flow September 2, / 21

15 While Loops while (<cond >) { <while -body > } This is easily translated into a conditional statement and goto: L1: if ( <cond > ) { <while -body > goto L1; } Geoffrey Brown, Bryce Himebaugh 2015 C Control Flow September 2, / 21

16 While Example... while ( count <= 10) { count ++; } L1: if ( count <= 10 ) { count ++ goto L1; } Geoffrey Brown, Bryce Himebaugh 2015 C Control Flow September 2, / 21

17 L1: if ( <cond > ) { <while -body > goto L1; }... L1: if (!<cond >) goto L2 <while -body > goto L1: L2: Simple Optimization We can get improved performance by a simple rearrangement: if (!<cond >) goto L2 L1: <while -body > if (<cond >) goto L1 L2: Geoffrey Brown, Bryce Himebaugh 2015 C Control Flow September 2, / 21

18 For Loops for ( <init > ; <cond > ; <update > ) { < for -body > } Which is equivalent to: <init > if (!<cond >) goto EXIT L1: <for -body > UPDATE: <update > if (<cond >) goto L1; } EXIT: Geoffrey Brown, Bryce Himebaugh 2015 C Control Flow September 2, / 21

19 Continue and Break continue and break are implemented as simple goto statements. goto EXIT; // break goto UPDATE; // continue Geoffrey Brown, Bryce Himebaugh 2015 C Control Flow September 2, / 21

20 Switch Statements Consider the simple switch statement: switch(x) { case 1 : x = 2; break case 2 : x = 3; break default: x = 4 } cmps r0, x == 1 bne c2 movs r0, 2 b end c2: cmps r0, 2 bne default movs r0, 3 b end default: movs r0, 4 end: Geoffrey Brown, Bryce Himebaugh 2015 Switch Statements September 2, / 21

21 Procedure Calls Procedure calls in the Cortex-M0 are implemented with a branch and link instruction. This is similar to the branch instruction presented previously, but, as a side-effect, it saves the address of the following instruction in the link register (LR). bl pc = label, lr = next inst. In previous examples we ve used the bx (branch exchange) instruction to return from procedures without explanation. Its function is to move the contents of a register (in this case the link register) to the program counter. bx pc = lr Geoffrey Brown, Bryce Himebaugh 2015 Procedure Calls September 2, / 21