Combine conversions 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

combine-conversions.sig combine-conversions.fun

Details and Notes

It processes each block in dfs order (visiting definitions before uses): * If the statement is not a PrimApp Word_extdToWord, skip it. * After processing a conversion, it tags the Var for subsequent use. * When inspecting a conversion, check if the Var operand on 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:

  • Case W3 <= W1 <= W2: 

x_3 = Word_extdToWord (W1, W3, {signed=_}) x1
because `W3 <= W1 && W3 <= W2`, just clip `x1`

  • Case W2 < W1 <= W3:

  • Case 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