CombineConversions is an optimization pass for the SSA IntermediateLanguage, invoked from SSASimplify.

Description

This pass looks for and simplifies nested calls to (signed) extension/truncation.

Implementation

Details and Notes

It processes each block in dfs order (visiting definitions before uses):

  • If the statement is not a PrimApp with Word_extdToWord, skip it.

  • After processing a conversion, it tags the Var for subsequent use.

  • When inspecting a conversion, check if the Var operand is also the result of a conversion. If it is, try to combine the two operations. Repeatedly simplify until hitting either a non-conversion Var or a case where the conversion cannot be simplified.

The optimization rules are very simple:

x1 = ...
x2 = Word_extdToWord (W1, W2, {signed=s1}) x1
x3 = Word_extdToWord (W2, W3, {signed=s2}) x2

  • If W1 = W2, then there is no conversions before x_1.

    This is guaranteed because W2 = W3 will always trigger optimization.

  • Case W1 <= W3 <= W2: 

    x3 = Word_extdToWord (W1, W3, {signed=s1}) x1

  • Case W1 < W2 < W3 AND ((NOT s1) OR s2): 

    x3 = Word_extdToWord (W1, W3, {signed=s1}) x1

  • Case W1 = W2 < W3:

    unoptimized, because there are no conversions past W1 and x2 = x1

  • Case W3 <= W2 <= W1 OR W3 <= W1 <= W2: 

    x_3 = Word_extdToWord (W1, W3, {signed=_}) x1

    because W3 <= W1 && W3 <= W2, just clip x1

  • Case W2 < W1 <= W3 OR W2 < W3 <= W1:

    unoptimized, because W2 < W1 && W2 < W3, has truncation effect

  • Case W1 < W2 < W3 AND (s1 AND (NOT s2)):

    unoptimized, because each conversion affects the result separately